spectrometer, and to Kim Tan for his support with the synchrotron radiation experiments. ... Canning, Geoff Hi& Kim PoUard, Mary-Anne MacDonald, Wei Hong, ...
Photoelectron Spectroscopy of Organornetallic Compounds
by
Jingcun Wu
Department of Chemistiy
Subrnitted in partial fulfilment of the requirements for the degree of
Master of Science
Faculty of Graduate Studies The University of Western Ontario
London, Ontario
May, 1997
8 Jingcun Wu 1997
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ABSTRACT
High resolution gas phase photoelectron spectra are reported for a series of trimethylphosphme substiMed twigsten carbonyls and cyclopentadienyl derivatives of indium (CpIn) and thallium (CpTI). The advantages of using monochromatic synchrotron radiation
PES for studying the electronic structure of organometallic molecules are demonstrated. For esch of the substituted tungsten complexes, al1 electronic levels fiom valence to inner-valence end core levels can be studied in one spectrurn with high resolution. The inner valence and core level spectm can be interpreted based on cornparison with published results.
Better resolution has been achieved in our newly obtained He 1 spectra of the valence level and W 5d regions of these complexes. Spin-orbit splittings, ligand field splittings, and vibrational structures are observed in the spectra of both W Sd and W 4f regions. Ligand field splittings on both the W 5d and W 4f levels increase in the order of W(COI6 = foc-
W(CO),(PM%),
4
W(CO),PMe, s cis-W(CO),(PM%), < Irm-W(CO),(PMeJ,.
Because
phosphine is a stronger o donor and weaker K acceptor than CO, al1 the metal and ligand
orbitals shift with different degrees to lower energies when CO is substituted successively by phosphine. Linear binding energy shifl trends are found in both core and valence levels of
tungsten and phosphorus ionizations, which confirm the ligand additivity predictions for these complexes. The corevalence ionhion correlation principle can be illustrated by comparing
the binding energy SM data betweencore and valence levels. The phosphorus 2p spinsrbit components of the phosphine substituted complexes have been resolved for the first tirne. To resolve the major controversial among theoretical treatrnents on the electronic structure and bonding of CpIn and CpTl, a photoelectron spectroscopie study with variable
photon energies has been carried out. The experimental results, especially the variations in band intetlsity as a function of photon energy, confirm our assignments of the spectra which are in good agreement with the results of both previous PES studies and Xa-SWand SCF caîculations. Due to the high resolution of our spectra, the vibrational structure in the lowest ionizations has bem resolved, and the broadening on the metal d levels caused by ligand field spiittings has been observed. In addition, the shake-up structures of metal d levels and core
level TI 4f have been studied for the first tirne with synchrotron radiation.
ACKNOWLEDGEMENTS 1w d d üke to take this oppomuiity to thank my supervisors, Dr. G.M.Bancroft and
Dr. RJ. Puddephatt, for th& guidance, encouragement, patience and fnendship, which have
made my study and research here possible. 1 am very pitefiil to Xiaorong Li and YongFeng Hu for their continued assistance in
my research, to Doug Hainine for his technicd assistance with the ESCA photoelectron spectrometer, and to Kim Tan for his support with the synchrotron radiation experiments. 1would also ike to express my Sncere thanks to my fnends and CO-workersfor their
support: DrXping Zhang, Dr. Sam Choi, Dr. Hilary Jenkins, Dr. Lou Rendina, Dr. Mike Scaini, Mike Irwin, Greg Spivak, Joshi Kuncheria, Daniel Legrand, Marina Fuller, Greg Canning, Geoff Hi& Kim PoUard, Mary-Anne MacDonald, Wei Hong, Jayasree Sankar, Cliff
Baar, and Michael Janzen. It is these fiiends, CO-workersand the supervison who have aeated and maintained the acadernic and fnendly atmosphere which have made my work in Western rewarding and enjoyable.
1 thank Dr. P.A.W. Dean, Dr. M.J.Stillman, Dr. N.C.Payne, Dr. N.S. McIntyre, Dr. RJ. Puddephatt, Dr. W.N. Lennarâ, Dr.O.L. Warren, Dr. K. Grifnths, Dr. J.G. Shapter,
Dr. A. Bassi, Dr. M.S.Workentin, and Dr. D.McConville for their instructions. 1 thank Dr. T.K.Sham, Dr. R.K. Chan, Dr. M.C.Usselman, Dr. R.G.Kidd, Dr. J.D.Talman, and Dr. P.A.W. Dean for their advice. 1 am gratefiil for the h c i a l support provided by the University of Westem Ontario
over the term of my studies. Finaüy, 1wouid like to tbank rny parents and my younger sister for their understanding
and support, with special thanks to my wife Chunning Li for her love and encouragement. This thesis is dedicated to my dear father Yuguo Wu and my late mother Qinxiou Li.
TABLE OF CONTENTS
Page
CERTIFICATE OF EXAMINATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .ii ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iii ACKNOWLEDGEMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .iv
TABLE OF CONTENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .v
. LIST OF TABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii LIST OF FIGURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vüi LIST OF ABBREVIATIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .x CHAPTER 1 Introduction 1.1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 1.2.1. Basic PMciple . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 1.2.2. Studies of Organometallic Compounds by Gas Phase PES . . . . . . . . . . . . . . . . .6 Ionkation (or Binding) Energy Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6 Splitting Effects and Fine Stnicture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8 Shake-up and ûther Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 Band Intensities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 1.2.3. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18 1.3. Outline of the Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18 1.4. References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 1.2.
Gas Phase Photoelectron Spectroscopy
CaAPTER 2 Experimental 2.1.
Preparation, Rirification, and Introduction of Sarnples . . . . . . . . . . . . . . . . . . . 23
2.2.
Recording the Photoelectron Spectra with Helium Light Source . . . . . . . . . . . . 25
2.3.
Recordhg the Photoelectron Spectra with Synchrotron Radiation . . . . . . . . . . 28
2.4.
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .30
CHAPTER 3 Photoclectron Sptctra of Trimethylphosphine Substituted Tungsten Carbonyls
3.1.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33
3.2.
Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35
3.3.
Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37
3.3.1. General Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37 3.3.2. ValenceLevelWSdandCoreLevelW4f . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 3.3.3. Higher Energy Spectra and Phosphorus 2p Bands . . . . . . . . . . . . . . . . . . . . . . 55 3.3.4. High Resolution Photoelectron Spectra of W(CO), NBD . . . . . . . . . . . . . . . . .60
3.4
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . 61
3.5.
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63
CHAPTER 4 Photdectmn Spectra of Cydoptntadicnyl Derivatives of indium(0 and Thallium~ 4.1.
Introduction . . . . . . . . . . . . . . . . .............................
4.2.
Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 .
4.3.
Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .71
67
4.3.1. General Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 . 4.3.2. Variable Energy Photoelectron Spectra of Cpln and CpTl . . . . . . . . . . . . . . . . 72 4.3.3. The Shake-up Satellites of the Metal d Levels and Tl 4f Bands . . . . . . . . . . . . 88
4.4.
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .98
4.5.
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 .
CaAPTER 5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104 APPENDIX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 VITA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .113
List o f Tables
Table 2-1
The sublimation temperatures melting points, and references of the organometaflic compounds studied in this work . . . . . . . . . . . . . . . 25
Table 2-2
Worbg parameters for recording the PE spectra of the studied compounds with helium light source . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
-
Table 3 1
Binding energies &), widths (Wd,and assignments of the inner-valence and core level spectra of W(CO),(PMe&
Table 3-2
(n = 1
- 3) . . . 4 1
Band positions (eV), widths (eV), assignments, spin-orbit coupling constants (0,ligand field splittings (A = b, - e or ba - e,), average binding energies (eV), and their shifts (eV) relative to W(CO),
in W Sd spectra of the listed complexes. . . . . . . . . . . . . . . . . . . . . . . . . 46 Table 3-3
Fitting parameters of W 4f spectra of the listed complexes . . . . . . . . . . 50
Table 3-4
Phosphorus 'lone pair' or o(W-P) and phosphorus 2p ionizations in W(CO),(PMeJn
Table 4- 1
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58
Binding energies (4)and relative intensities (Ir)of the peaks in CpIn by He 1, He II, and SR (at 80 eV) PES and calculated binding
energies (4)and eigenvalues ( q ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . .76 Table 4-2
Binding energies (&)and relative intensities (Ir)of the peaks for CpTl by He 1, He II, and SR (at 80 eV) PES . . . . . . . . . . . . . . . . . . . . . . . . .77
Table 4-3
Binding energies (4)and widths (FWtPul) of metd nd bands
for TU(, TlCp, and InCp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .92 Table 4-4
Derived crystai field parameters (eV) for TlX, WI, TICp, and InCp . . . 93
Table 4-5
Shake-up energies (A) and widths of CpIn and CpTl . . . . . . . . . . . . . . . 95
List of Figures
Figure 1-1
Block diagram showing the arrangement of the principal parts of a photoelectron spectrometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 1-2
Photoelectron spectra of W(C%, (a) broad-scan spectrum with synchrotron radiation source; (b) high resolution spectrum of W 5d region 4 t h He 1 source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4
-
Figure 3 1
Broad-scan PE spectra of (a) W(COl6, @) W(CO),PMq,
(4 cis-W(C0)4(PM4%)2, (4tr~-W(C0)4(PM%),, (e)/ac-W(CO),(PMe&, Figure 3-2
and (f) W(CO),NBD . . . . . . . . . . . . . . . . . . . 40
He I valence level spectra of (a) W(CO),PM%, @)trans-W(CO)4(PM&,
cis-W(C0)4(PM&
and (d) W(CO),NBD. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42 Figure 3-3
He 1 spectra of W 5d levels in (a) W(CO),PM%, (b) C~.S-W(CO)~(PM~J, (c) ~~L~zs-W(CO),(PM~),, (d) W(CO),NBD . . 45
Figure 3-4
High resolution W 4f core level spectra of (a) W(CO),, @) W O ) , + W(CO),PM%,
(4 cis-W(CO)4(PM%)*,
(dl ~ ~ s - W ( C O X ( P M s ) ,(e)fac-W(COX(PMe,)3, and (f) W(CO)4NBD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49 Figure 3-5
A diagram showing the correlation between the ligand field
splitting ofW 5d bands and the width ofW 4f bands. . . . . . . . . . . . . . . S i
Figure 3-6
Shift c o m p ~ s o ndiagram for tungsten and phosphorus binding energy shifts: W 5d (valence), W 4f (core),
P 'lone pair' (valence), and P 2p (core) . . . . . . . . . . . . . . . . . . . . . . . . . 52 Figure 3-7
-
Core valence shifi correlation for tungsten and phosphorus
-
ionizations: W 5d W 4 î and P 'lone pair' Figure 3-8
- P 2p . . . . . . . . . . . . . . . . 53
High resolution PE spectra: (a) broad-scan of c~s-W(CO)~(PM~&
at 100 eV, (b) phosphorus 2p bands in cis-W(CO),(PMe& at 152 eV, (c) W 4f bands and the second order bands of
phosphorus 2p at 101 eV, and (d) W 4f bands and the second order bands of phosphorus 2p at 102 eV . . . . . . . . . . . . . . . . . . . . . . . . 59 Figure 4- 1
High resolution broad-scan photoelectron spectrum of InCp at 80 eV . . 73
Figure 4-2
High resolution broad-scan photoelectron spectrum of TlCp at 80 eV . . 74
Figure 4-3
Valence level PE spectra of (a) InCp (He I), @) InCp (He II), (c) TlCp (He I), and (d) TlCp (He II)
Figure 4-4
. . . . . . . . . . . . . . . . . . . . . . . . . . 75
Variable energy photoelectron spectra of InCp at (a) 80 eV, (b) 130 eV, (c) 140 eV, (d) 150 eV, (e) 160 eV, ( f ) 180 eV . . . . . . . . . 78
Figure 4-5
Variable energy photoelectron spectra of TlCp at (a) 80 eV, @) 130 eV, (c) 140 eV, and (d) 160 eV . . . . . . . . . . . . . . . . . . . . . . . . 79
Figure 4-6
Photoionization cross section for atornic C 2s, C 2p, In Ss, In 5p, In 4d, Tl 6s,Tl 6p, and Tl Sd subshells
Figure 4-7
.. ...... ........ . . . .. . ...
80
Variation in relative intensity of band A and band B as
a function of photon energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Figure 4-8
Variation in relative intensity of band C and band D as
a fùnction of photon energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 Figure 4-9
Variation in relative intensity of band C, D and E as a fùnction of photon energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Figure 4-10
PE spectra of metal d region in (a) InCp (He II), @) InCp at 70 eV (SR), (c) TlCp (He II), and
(d) TlCp at 80 eV (SR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 Figure 4-1 1
Variation in relative intensity of the bands as
a function of photon energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 Figure 4- 12
Shake-up bands of In 4d and Tl 5d regions of (a) InCp and (b) TlCp . . . 94
Figure 4- 13
Variation of the intensity ratio of metal d bands (F+F') with their shake-up bands (H,+H,') as a function of photon energy . . . . . . . . . . . . 96
Figure 4- 14
Photoelectron spectrum of Tl 4f region in TlCp obtained at 280 eV . . . 97
List of Abbrcviations
PE
photoelectron
PES
photoelectron spectroscopy
SR UPS XPS
synchrotron radiation ultraviolet photoelectron spectroscopy
eV
electron volt (8065.73 cm -')
FWHM
full width at baif the maximum intensity
MO HOMO
molecular orbital
A0
atornic orbital
BE (or 4,) E
binding energy
BR
branching ratio
WC) SCF
spin-orbit coupling (splitting)
Xa-SW
Xa scattered-wave
PSD
position sensitive detector
CP
q5-cyclo pentadienyl
CpIn (or InCp)
cyclopentadienylindiumo
CpTl (orTlCp)
cyclopentadienylthallium(I)
Me
methyl
PM%
trirnethylphosphine
NBD
norbomadiene
X-ray photoelectron spectroscopy
highest occupied molecular orbital
ionization energy
self-consistentfield
Chapter 1
Introduction
1.1.
Introduction The chemistry of organometallic compounds is fundamental to many organic and
inorganic syntheses, cataiysis and surface reactions, and bioinorganic processes (or biological systems). In order to understand this chemistry in ternis of the basic electronic structure
mon thaî control electron distribution, bonding, geometry, and reactivity or stability of the cornpoundq many experimmtal and theoretical studies by photoelectron spectroswpy (PES)
have been camed out since the pioneering works in 1960.' in the past, photoelectron spectroscopy (PES) mainly relied on laboratory light sources and was classified into two areas based on the sources used: (1) molecular photoelectron spectroscopy, or more cornmoniy cailed, ultraviolet photoelectron spectroscopy (LPS), and (2) X-ray photoelectron spectroscopy (XPS). UPS is used to study the valence bands. The most useful and cornrnonly utilized UV light sources are He I (2 1.2 eV) and He Il (40.8 eV) resonance lines. One of the advantages of using ü P S is its high resolution. The variations of band shape and
fine structure due to various spiitting effècts and vibrational couplings can be observed in the
valence spectra of many molecules, *
S U C ~as the W
Sd spectra of W(CO),(PMq),
in this
study. XPS is usuaily used to study the core level binding energies of atorns in molecules. The lllnited resolution of X-ray sources bas restricted its application for valence band studies.
The most versatile photon energy source is synchrotron radiation (SR), which has the
2
advantages of high htensity, high resolution (using a monochrornator), continuously tunable radiation, and wide spectral range covering fiom the vacuum W to soft X-ray regions. In the last ten years, monochromatic synchrotron radiation (SR) has been used in combination
with He I / He II photon sources for gas phase photoelectron spectroscopic studies of inorganic and organometallic compounds, in which information about both energy and intensity, as well as fine structure can be obtained. Such studies have greatly increased the power of PES. Photoelectron spectroscopy has now become one of the most direct
experimentd methods for probing the energy levels of the electrons in substances, and characterizing their electronic str~cture.'~ ' This thesis will focus on the gas phase photoelectron spectroscopic studies of organornetallic compounds. There have been a lot of PES studies on transition metal carbonyls and their
'
denvatives, possibly due to their typical synergic bonding characters and high volatility.'*
The Group lII cyclopentadieny1derivatives MC&
M = In or Tl, are of interest because they
are rare examples of 'halfsandwich' organometallic rnole~ules.~ In this work, the electronic
structure of a series of phosphine substituted tungsten carbonyls and the cyclopentadienides of indium (I) and thallium (1) are studied with a combination of synchrotron radiation and He 1 / He II photoelectron spectroscopy. Special attention and interest are focused on studying the effects of ligand replacement on the PE spectra of the phosphine substituted tungsten carbonyls. 1.2.
Gis Phase Photoelectron Spectmscopy
1.2.1. Basic Principle
When photons of sutncient energy interact with a molecule, ionization can occur with
ejection of electrons (so-ded photoelectron). M+hu-.M++e'
(1)
The photon energy, hu, can be transferreû to an electron, enabling it to overcome the electron
binding energy,E, (the force which binds the electron within a molecule), and giving it kinetic energy, E, . Based on Einstein's photoeiectric effect:
&=hue&
(2)
Since the photon energy (hu) is known in a certain photoelectron experiment, and the
kinetic energy (&) is measured accurately by an electron energy analyzer (see Figure L I ) , the electron bhding energy (4)can be detennineâ by experiment (if the ho is large enough,
al1 electrons fiom valence, i~er-valenceand core levels of a molecule can be ejected, and their 4,'s can be detennined by the photoelectron experirnent). Photoelectron spectroscopy (PES) is an expenmentai method for studying these photoelectrons, their energy levels and
their relationship with the electronic structure of a molecule. A photoelectron (PE) spectrum consists of a plot of the 4,or E, versus intensity in
electron counts per second (or the number of photoelectron with a certain energy), which
represents the energy distribution of photoelectrons in a molecule. According to convention, the spectra are ploned with E, increasing fiom lefi to right, and E, increasing fiom right to left (see Figure 1-2). A PE spectrum can be relatd to the molenilar orbital energy pichire
S' of a molecule by K W P ~ Q I ~%wem
' which States that the ionization energy (E)is equal
to the negative of the self-consistent field (SCF)orbital energy (-ej ).
IE = -ej
(3)
COMPUTER
r
RECORDER
1
PHOTON SOURCE
TARGET CHAMBER
bI
ELECTRON E N E R G Y ANALYZER
F-1
l
DETECTOR
Figiisc I - 1. Block diagrani sliowiiig tlir arrangement ol'tlir priiicipal parts of r7 photoelectron spectronieter.
Figiire 1-2. Photoelectron spectra of W(CO),. (a) broad-scan spectriirn witli synchrotron radiation source: ( b ) Iiigli resoliition sprctsiini of W 5d regioti witli He 1 soiirce.
5 Although various approximations are involved in this theorem and quantitative predictions ofIE are not accurate, this theorem has been used widely in the interpretation of PE spectra (such as in predicting the number, type and energy of primary PE spectral bands of a moleaile). The most signaficant deduction from the correlation of K m p r n ~ l'l Theorem ~ and m o l d a r orbital energy diagram of a molecule is that there is a one-to-one conespondence between the primary band features of a PE spectrum and the occupied molecular orbitals of
a closed sheU moleaile. In its sirnplest form, one m o l d a r or atomic orbital gives rise to one spectral band, which provides us the best expenmental method of obtaining molecular and atomic orbital binding energies. For example, in the PE spectnim of W(CO), 1-Za), band 1 ( at
" ( Figure
- 8.5 eV) aises from the ejection of electrons fiom the valence W 5d
orbital (i.e. the t, orbital, see the qualitative molecular orbital diagram for W(CO), in
Appendix A). Band 2 is due to ionizations of electrons fiorn mainly CO 5 0 and 1n orbitals.
Band 3 is from ionization of CO 40 orbitals. Bands C, and C, are resulted from core level W 4f electrons. The bands from S to D are rnainly ligand-based inner valence orbitals, and they are ofien ditticult to interpret accurately for organometallic compounds because of the
orbit overlapping and shake-up interactions in t his region.
When analyzing the PE spectrum of a molecule, dl band features such as energy, width, shape, resolved fine structure and relative intensity, as well as their changes with different photon energies and chemical variations, should be taken into account.
Helium light sources can provide enough energy for ionization of the majority of valence electrons. The high resolution of He 1PES is critical for the studies of the valence electrons in organometdic compounds.* Observations of resolved fine structure due to
6
vatious spiitting effects and vibrational stretchings have not only increased Our understanding of moleailar electronic structure, but also greatly assisted Our assignrnent of PE spectra for the studieà molecules (see Figure 1-Pb" and Chapter 3). However, in order to study core
level electrons and obtain both energy and intensity information about the ionization processes of a molecule, the use of continuously tunable Synchrotron Radiation (SR) is ofien necessary, and has proven to be very powerful for studies of the electronic structure of organometdîic compounds in the last ten years (see the following section and references cited). 1.2.2, Studies of Organometallic Compounds by Gas Phaae PES As mentioned previously, al1 the characteristic features of a PE spectral band
(binding energy, width, band shape, resolved fine structure, and relative intensity) ought to be considered when studying a PE spectrum, because al1 the features are sensitive to the electronic structure of the m d d e . The ionization energies (IE 's) or binding energies (4,'s) are most signifiant in terms of trends between related molecules. The other band features
can directly or indirectly reveal information about the electron localization on the metal and ligands, the interactions between metal and Ligands (bonding, or nonbonding nature of the orbitalq splittings and vibrations), and the different variations in photoionization cross section between metal and ligand orbitais.
Ionization (or Binding) Energy Trends.
P@s
the most important use of PES
is to obtain IE's or l$'s of atornic and molecular orbitals based on equation 2 (6= hu - Q. Although the IE's or 4 ' s are characteristic of the electrons in a molecule which are independent of the photon energy, they do Vary over a certain range depending on the
7
chernical variations on the molecule. The shift in an IE or E, between electronically or chemically related molecules is an especially revealing and usefiil feature of the electronic structure. In sorne cases. the correlation of IE 's or 4 ' s between related molecules cm assist in the assignrnent of spectra, such as in the comparison of the IE's or 4 ' s of published
W(C% spectra with those of phosphine substituted tungsten complexes, as illustrated in chapter 3. In other cases,the electronic perturbations caused by chernical group substitutions produce identifiable IE or E, shifis which r e v d the localized or delocalized character of the electronic states and the 0uiuidity of charge in the system. In these cases, the IE 's or Eb7s cm serve as a good m a u r e of the substituent effects and can assist in monitoring the effects of
chernical variation on a molecule. For exarnple, the IE or E, shifls in both valence and core levels of organometallic compounds can be very useful in studying the a-donor and rracceptor properties of ligands, and the additivity of the o and x effects of ligands on a metal
tenter.' The ligand additivity rnodel states that valence metal orbital IE's or 4 ' s are shified in a linear way as one kind of ligand in a moleaile is substituted successively by another kind
of ligand.' Although this rnodel was proposed initidly for valence rnetal ionizations, it has proved to be valid for both valence and core level ionizations by several studies (see chapter 3 and cited references). For instance, the series of M(CO),(PMe&,
complexes (M= Mo and
-
W, and n = O 3) have been studied in detail (see chapter 3), in which both valence and core
metal IE's or &'s are shifteâ linearly to lower energies with each step of ligand replacement because the total donor ability (O-donationminus n-acceptance) of PM% is greater than CO
(i.e. PM% is a stronger a-donor but weaker n-acceptor than CO). A third application of ionization energy trends is baEed on the correlation of core and valence IE or E, shifts, which
8
allows the differentiation of the influences of bonding / overlap and charge potential contributions to 1 . 5or &S.
The principle of core-valence ionization correlation states that,
when comparing huo related molecules, the binding energy shift of a nonbonding valence orbital ofa certain atom is approximately eight tenths of the core binding energy shift for that atom between the two molecules, i.e. aEMdma)/A&-)
= 0.8 &m.
A value of'
> 0.8 indicates the contribution of bonding /overlap interactions to the
valence SM. The reason why the core level E,, shift is larger than the valence E, shift is that, according to ~ollf or Lichtenberger,
the core &'s
are mainly determined by the charge
distributions and relaxation energies, whereas the valence 4's are affected by these factors as weii as aD aspects of chernical bonding (Le. valence &'s are more sensitive to the bonding effects than core G's)Although organometallic compounds seldorn have the strictly nonbonding valence orbital, the correlation values obtained from the above M(CO),(PMq),
complexes have been shown to be in reasonable agreement with the principle's predictions of 0.8 i 0.1. The examples of carbonyl and phosphine complexes mentioned above demonstrate how PES can provide detailed and sometimes quantitative information on each individual interaction of a iigand with a metal center. The interactions are observeci directly in terrns of stabhtion or destabilization of the orbital IE 's. The interaction of the ligand a orbital, or
r orbital is a separate effect." The binding energy shüt (4) is dependent on the overall charge potentiai on a orbital. We can apply these pnnciples to the studies of various metalligand interactions in organometallic compounds. Splitting Eff'ts and Fine Structure.
" PE spectral bands often exhibit fine
9
structure resulted fiom various splitting and vibrational effects which are associateci with the molecuiar ion states. Even though such structure is often not well resolved, it can affect the band shape. Such fine stnicture not ody provides detailed information about the molecular electronic structure but also offers considetable help in the interpretation and assignrnent of the PE spectra. However, in order to study these fine structure features, high resolution PES
is ofien needed. Spin-Orbit Spliffing. We often see from the PE spectra that one band is split into two. The major reason for this splitîing is due to the coupling of spin angular momentum (S)
with orbital angular momentum (L) (so-called spin-orbit coupling) based on J
=Lk
S.
Removal of an electron fiom a filled p, d, and f orbital, leaving the orbital with an
unpaired electron, always gives a doublet in the PE spectrum. For example, the P 2p and W 4f doublets in phosphine substitutd tungsten complexes (sec chapter 3), and the In 4d, TI 5d and Tl 4f doublets in InC5H5and TlC5H5(see chapter 4) have been resolved in this work (because S = W 2 ; L = 1, 2, and 3, respectively for p, d, and f orbitais). The magnitude of
spin-orbit splitting is approximately proportional to the square of the atomic number of the atom for the valence shells of a many-electron system;1° a larger splitting is expected for a
second or third row transition metal valence d orbital, and this is somethes called the heavy metal effea.21'It is important to notice that for core levels, the spin-orbit splitting is not
chemicdly sensitive compared with that for valence levels. The splitting for a given atom
"
increases 60rn valence to a r e level (e.g. see chapter 3 for the spin-orbit spütting values of
W 5d and W 4f). The spborbit coupling theory was first used by Hal to interprete PE spectra of
10
transition metal systems, which 1 4 to a definitive assignment for the spectra of XRe(CO), species? This theory has been shown to be very useful for the assignment of metal d orbitals of the second or third row transition metal complexes.*
"
Ligmd Field Splitting. Ligand field effects on the metai centet can dso lead to spiittings in metal orbitais, which are often shown as split or broadened spectral bands in the
PE spectra. Crystal field and ligand field theories have been used to account for the effects on the PE spectra. l2 Previous studies have shown that the ligand o-donor and x-acceptor effects on the valence d levels can be sepamed. The biiding energy shifis of the metal orbitals with ligand
substitutions depend on the total donor ability (a-donation minus x-acceptance) of the substituted ligand relative to that of parent ligand, while the ligand field splittings of the metal
orbitals depend only on the relative x-acceptor ability of the substituted ligand and the parent ligand. The magnitude of the spütting is proportional to the difference in x-acceptor abilities
of the two different kinds of ligands. This has been confirmeci by the study of a series of LM(CO), complexes (M = Cr, Mo and W; L = PEt,, PM%,P(NMeJ,, P(OEt),, P(OMe),,
PF,)?
The ability of the ligands to split the
e and
4 components decreases in the above order (fiorn left to right), showing that the
orbitals of the parent hexacarbonyl into the
n-acceptor ability increases in the same order. This ligand field splitting effect will dso be discussed in chapter 3 for compounds of W(CO),(PMq),
-
where n = O 3.
Ligand field splittings on core levels of main group compounds and metai surfaces from photoelectron spectra have been reviewed.12 On core d levels of non-cubic compounds
such as (CH3)$d and TU[ (X = Cl, Br, and I), the d, level splits into two and the d, level
11
spüts into three. Simiiarly, for the metal 4f orbitais in complexes of Os(CO),L (L= CO, and PMeJ,"
lSaD
Re(CO)&(X = Re(CO), CI, Br, and I),% l5 and W(CO),(PM%)n (chapter 3),
the f,, level splits into three and the f , level splits into four. This latter effect results in brodening of the spectra of metal 4f levels in these complexes. The crystal field Hamiltonian for the ligands interacting with the core d or f levels is :
-
H = Cp [3L: - L(L+l)] +C:[35L: 30L(L+ 1)L: -25L:- 6L(L+1)+ ~ L * ( L + I] ) ~(4) where, the nonaibic C: term dominates over the cubic C,"term. It is possible to diagonize
the Hamiltonian math and obtain five equations for the four unknown E, C:, Ct and the
spin-orbit splitting A.& This splitting effect wil1 be examined in more detail when discussing the In 4d and Tl 5d spectra of InCp and TlCp compounds (see chapter 4). fibrutionai Splitfiing. The principle of vibrational splitting in the valence PE spectra has been described pre~iously.~ 'O Different electron transitions fiom the ground state of a
m o l d e (v = O for most molecules) to a series of vibrationai energy levels of the molecular ion state, govemed by Franck-Condon rule, usually lead to the vibrational structure in both valence and core level PE bands. For organometallic compounds, observation of wellresolved vibrationai fine structure is not often possible due to the small metal-ligand vibrational fkquencies, or because severai difEerent vibrational progressions are excited by
a single ioni~ation.~ Therefore, high resolution PES is required for studying these fine structure featuns. For valence bands, vibrationai progressions are often related to ionizations
from bondhg or antibonding MO'S.' Even for unresolved bands, the band shape can indicate to some extent the bonding nature of the vacated orbital, e.g. narrow bands are associated
with nonbonding orbitals and broad bands with bonding (or antibonding) orbitals. However,
the observation of core level vibrational splittings cm be best interpreted by considering the core quivalent model, which states that when a core electron is removed fiom an atom or m o l d e , the valence electrons relax as if the nuclear charge of the atom had increased by one unit. l3 According to this model," the properties of a molecular ion with a core hole are approximated by the moldar with the Z+1 atom. For example, the core-equivalent species for core ionized W(COI6 is Re(CO),'.
'
In this work, aii the spütting e f f i s and fine structure features mentioned above have been observed in the study of phosphine substituted tungsten carbonyls (see chapter 3).
Jahn- Teller SpIitting. In addition to the above splitting effects, sometimes Jahn-
Teller qliftingcm be important in yielding extra peaks in the valence band. The Juhn- Teller h o r e m states that a non-linear molecule in a degenerate electronic state is unstable towards
distonions which remove the degeneracyZh'O Jahn-Teller spiitting often fùrther splits a spin-orbit split state. These splittings have been found in the PE spectra of metal d orbitals
of Fe(CO)514and Os(CO),.' Shabup and Othtr Eff'cfa
Extra bands are sometimes seen on the low kinetic
energy (hi@&)side of a core l t d or vaience ievei band, such as in the spectra of W(CO)6.'5 These bands, so-calleû shake-up satellites, are oflen broader than the main bands '' and can be illustrateci by the foliowing processed6 As the result of a vacancy formed in a given orbital due to ionisations, the electrons in the sarne orbital or other orbitals see a change in the effkctive nucleat charge due to an alteration in the electron screening. This change in effective nuclear charge can give rise to an excited state in which an electron may undergo
a transition to a discrete state -shake-up which is shown as low A?$satellites or it may go to
the continuum (shake-on) state. The shake-up peaks of metal d orbitals in InC,H, and TIC& have been seen using
high resolution PES with SR and wiU be discusd in chapter 4. The intensity of the shake-up band S observed previously l5 in W(COI6 decreases wit h each CO being replaced by PM%, which further confimis that this shake-up results fiom a CO valence band between 13
- 15 eV (see chapter 3). The extra peaks created by other eff'ects, such as the "self-ionization" peaks (He*) and
the peaks excited by He II P(48.37 eV) or He Ii y (SI .O1 eV) satellite lines of He Il emission are found in the He II spectra of InC& and TiC& (chapter 4). Most of these peaks overlap seriously with the main bands of the samples, which make it difficult to assign the spectra accurately. To overcome these problems and obtain the red spectra of sarnples, high resolution synchrotron radiation PES is used in this study (which can get rid of the excitation fiom He II satellite lines). The second-order ionization bands of P 2p spinsrbit components are also observed in the phosphine complexes (chapter 3). Band Intensities.
In addition to the information mentioned above, a PE spectrum
cm also provide information about band intensities. Aithough IE's or 6 ' s are characteristic
of the shidied moleaile and are independent of the photon energy, the band intensities depend on the angle of observation of the photoelectron with respect to the photon beam, the polarization of the photons, and the photon energy. If the electrons are observed at an angle 8 to the direction of the electric vector of a plane polarized light beam, the intensity I(8) is
given by e q ~ a t i o n : ~ ~
-
I(0) = a/4n[l + (P/2)(3cos2 0 1)]
(5)
14
where o is the total cross section integrated over al1 angles, and P, known as the anisotropy parameter, is the only parameter required to descnbe the angular distribution of the photoelectrons. However, for most studies on inorganic and organometallic compounds, the measurements of band intensities are perfonned at a "magic angle" (8, = 0,
= 8, = 54.7')
where the band intensity is directly proportional to the cross section ( the probabitity of photoionization to an ion state is nonnally designated as photoionization or photoelectron cross ~ection).~ The photon energy dependence of a band's intensity or cross section is characteristic of the nature of the ionized (or vacated) molecular orbital (MO). This nature depends on the types and the compositions of atornic orbitals (AO's) which make up the
MO." Thus, intensity or cross section studies cm provide more ~sefulinformation about molecular electronic structure and give a firm basis for PE band assignrnent.
The d
y studies on band intensity were based on the cornparison of He 1 and He II
spectra of the studied molecules. It had been shown that the intensity of the valence bands fiom ligand orbitals (e.g. the s and p orbitals of C, N, O, P, and S. especially C 2p orbital for organometaiiic compounds) d e c f w with photon energy increasing fiom 2 1.2 eV (He 1)to 40.8 eV (He II), while the band intensity of valence d orbitals of most transition metals
inmeases drarnatically with photon energy changing within the same region. This empirical relationship of relative He 1 and He II ionhation band intensities had been used to interpret the valence band spectra of many organometaüiccompounds." However, this method was
lllnited to onS, two separate photon energies, and in this energy region ( around 40 eV) there
exist other variation effects on band intensities, such as shape resonances and interchannel cou pling which may lead to rnistakes in the assignrnent of spectra.' Therefore, in order to
15
obtain diable and definite assignments of PE spectra, the variable photon energy PES with SR should be applied, which provides the intense tunable source of photons required to study
the continuous variation of atomic and molecuiar photoionization cross sections within a wide range of photon energies. The applications of variable energy PES in the interpretations of cross section variations and in the PE band assignmnts rely on the Gelius model," which assumes that the
cross section of a molecular orbital (MO) is mainly determined by the atomic orbital components of that molecular orbital,
q ZP,,oy0 where, qMo is the cross section of the jth MO,
(6)
oAt0are the atomic cross sections for al1
orbitals of atoms Ai in an MO, and PA,is the "probability" of finding in the jth MO an electron belonging to the atomicA i ohitai. This means, for organometallic compounds, the cross section variation of non-bonding metal d and ligand carbon 2p orbitals behave like their
atornic countqarts, and those of the bonding MO'S behave in an intermediate manner. This mode1 has been used successfully in many recent variable energy PES studies of organometallic wmpounds.
'D
'
Several characteristic features of MO'S can be obtained Born studying cross section
or intensity variations with photon energies. Most of these qualitative features can be
understood in terms of the interactions between the ionized (vacated) MO and the outgoing electron wave.' A Generd Trend in the P h t o i o n i d o n C m Section. A cross section is generally highest near the ionkation threshold, and afler that, it decreases with the increase of photon
16
energy, such as the ligand C 2p orbital cross section. When photon energy Uicreases, the kinetic energy of the electron increases and its wavelength decreases, the electron wave b m e s more oscillatory, the positive and negative parts of the dipole m a t h element with the ionized orbital tend to canal one another which lead to the rapid decay in cross section." However, for some metal d and f orbitals, there are other features superimposeci on the generai decay of the cross section, such as delayed maxima, Cooper minima, resonance effects
and so on. These features are characteristic of these metal d and f orbitals.
Cwper Minima
For orbitais whose radial wavefùnctions have a node (the number
of nodes = n 4-1), there is a minimum in their cross sections- so-called Cooper minimum,
such as for the 4d and 5ftransition metd orbitals. In contrast, other orbitals without a radial node (Is, Zp, 3 4 and 4f) do not show Cooper minima, such as C 2p and Ni 3d cross sections.
This Cooper minimum can be illustrated by a change in the phase of the initial state wave function which results in cancellation of the electron dipole transition moment to the final state wavefùnction at some energies. In previous studies, Li has used the Cooper minimum in the 4d ionization of Pd to assign the PE spectnim of Pd(q-Ca,), and show that the ionstate ordenng of this cornplex is different fiom that of ~ i ( q - C ~ H ~ ) , . l ~
Delayed Mmima. Orbitals with high angular momentum, such as the d and f orbitals of transition metals, of€enshow maxima in their cross sections some way above threshold. This is in contrast to s and p orbitals whose maxima (if they have one) tend to be near their threshold. The delayed maxima in the cross sections of d and f orbitais
can be explained by the larger centrifiigal barrier effects of the high angular momentum electrons?
17
Resonunce Eflects.
For nd (or nt) orbitals of transition metai complexes, great
cross d o n or band intensity changes may be obsaved at the corresponding i ~ enp r (or nd,
for nd-nfresonance) threshold. Such great variations in cross section or band intensity result
fiom indirect ionizations or resonant excitations, which can be illustratecl by a two-stage process: n p 6 n d x - n p S d n l - n p 6 n d x - ) + e -or
d i o n,nd9nf*' fx
-&'Onf'-1
.
+ e-
First, an inner np (or nd) electron is excited to one of the empty nd (or nf) orbital. Subsequently, an electron Ws back into the np (or nd) hole and an outer nd (or nf ) electron
is i o n i d - the d e d super Coster Kronig (SCK) transition. Therefore, an enhancement in the nd (or nf) orbital cross section may be observed in the region of the np (or nd) absorption. The large metd np
- nd resonant effects observed in some organometallic systems
have assisted the definitive assignments of metal d-based ionkations in the valence spectra.) For example, these resonance have aided the assignments of valence d bands in the complexes
of CpM(CO), (M = Mn and Re)" and Os(CO),PM%. The theories and appiications of these cross section features in variable energy
photoeltxtron spectroscopie studies of transition metal systems have been discussed in detail
in recent review articles.' In surnmary, the cross sections or intensities of d and f PE bands of gas-phase molecules show a number of characteristic features, including maxima at relatively low photon energies due to centrifûgal banier effkcts, resonant effects (Le. pronounced maxima and minima) at photon energies corresponding to resonant absorption
18
by inner shell eiectrons, Cooper minima at high photon energies for ionization from orbitals with radial nodes, and shape resonances. These features lead to highly different photon energy dependences between the cross section or intensity of rnetal d (or f) bands and that of ligand-baseû bands. Studying these difEerent features between metal and ligand bands can
not only lead to firm experimentaiiy based band assignments, but also can increase Our understanding of the interactions between metal and ligand orbitals within a molecule. The achievements in this area have been dernonstrated by the recent studies of inorganic and organometallic compounds with variable energy PES? ' 1.2.3. Conclusions Studying organometallic compounds by gas phase PES is not only desirable but is also
possible since volatile compounds cm be obtained either cornmercially or by synthesis. %y synthesizing a group of compounds (such as by keeping the same metal center and changing
the nurnber or type of the ligands, or by keeping ligand the same but varying the metals), the ligand substitution effects on the metal center (chapter 3) or the periodic trend of rnetal ionizations 4b can be studied. Wth the combined use of helium and synchrotron radiation sources, PES has become one of the most important and direct experimental methods for studying the electronic structure of organometaîüc compounds. 1.3.
Outline of the Thmis
This thesis is composed of 5 chapters. This first chapter offers a general introduction of gas phase photoelectron spectroscopy (PES), including the basic principle and some
important applications of gas phase PES in the study of organometallic compounds. Chapter 2 d e s c r i i the experirnental methods and working conditions used in this study of work.In
19
chapter 3, high resolution gas phase photoelectron spectra are presented for a series of trimethylphosphine substituted tungsten carbonyls. Spinsrbit splittings, ligand field effects and vibrational stmctures are observed in the spectra of both W 5d and W 4f regions. The
linear binding energy difi in both valence W 5d and core W 4f levels confirms the validity of ligand additivity principle. The wre-valence ionization correlation can be illustrated by comparing the binding energy shift data between valence and core ionizations. Chapter 4 discusses the study of InC,H, and TICsH5by PES. Our He 1 and He II spectra show better resolution than the previously reported. With synchrotron radiation PES, the shake-up satellites of metal d orbital are observed which are included in the broad-scan spectra of I n C a and nC&. The variation in the relative intensities of the spectral bands can be seen in the variable energy PE spectra. For the first time, the Tl 4f spin orbit components are resolved in this study. Chapter 5 provides some of the conclusions obtained from this study.
Referenca
(a) Lloyd, D. R.; Schlag, E. W . Inorg. Chem. 1969, 8, 2544. @) Evans, S.; Green, J. C.; Green, M.L. H. : Orchard, A. F.; Turner, D. W .Dismss. Fwarioy Soc. 1969, 47, 112.
(a) Lichtenberger, D.L.; Kellogg, G. E. Acc. C h .Res. 1987, 20, 379. @) Eland, J. H. D. "Photoeiechon Spctroscopy, An Inrrodirclion to Ultr~viofet
Photoeiectron Spctroscopy in the Gus Phase ", Buthenvorth & Co (Publishers) Ltd., Second Edition, 1984.
(a) Green, J. C . Acc. Chem. Res. 1994, 27, 13 1.
O>) Green, J. C . Encyciopedia of
Inorg. Chem. 1994, 6, 3257. (c) Bancroft, G. M.;Hu, Y. F (Review, to be published).
(a) Hu, Y. F. Ph. D.Dissertation, The University of Western Ontario, London, Ontario, Canada, 1996. (b) Li, X . Ph. D.Dissertation, The University of Western
Ontario,London, Ontario, Canada, 1995. (c) Yang, D. S . Ph. D.Dissertation, The University of Western Ontario, London, Ontario, Canada, 1989. (d) Dignard, L. M .
Ph D. Dissertation, The University of Western Ontario, London, Ontario, Canada, 1986.
(a) Yarborough, L. W.; Hall, M.B. I m g . C'Che. 1978,17,2269. @) Bursten, B. E.; Darensbourg, D. J.; Kellogg, G. E.; Lichtenberger, D. L. lnorg. Chem. 1984, 23, 4361. (c) Banaoft, G. M.; Dignard-Bailey, L.; Puddephatt, R J. Inorg.
Chem.1984,
23, 2369. (d) Bancroft, G. M.; Dignard-Bailey, L.; Puddephatt, R J. Inorg. Chem. 1986. 25. 367. (e) Pudde~hatt.R J.: Dinnard-Bailev. L.: Bancroft. G. M . Inorp.
21
Chim. Ac& 1985,96, L91. (f) Lichtenberger, D. L.; Kellogg, G. E.; Landis, G. H.
J. Chem.Phys. 1985, 83, 2759. (g) Lichtenberger, D. L.; Kellogg, G. E. Acc. Chem. Res. 1987,20,379. (h) Hall, M. B. J. Am. Chem. Soc.,1975,97,2057.
(a) Craddock, S.; Duncan, W.; J. C h .Soc., F w a h y Trmts, 2, 1978, 74, 194. @) Egdell, R. G.;Fragala, 1.; Orchard, A. F. J. Electron S'ctrosc. Relat. Pherom.
1978, 14, 467. Koopmans, T. Physica, 1934, 1, 104. Bursten, B. E. J. Am. Chem. Sm. 1982,104, 1299.
(a) Jolly, W. L. AccChem. Res. 1983, 16, 370. @) Beach, D.B.; Jolly, W. L. Inorg. Chem. 1986, 25, 875. (c) Jolly, W. L. C h .Phys. Lett. 1983,100, 546.
(d) Jolly, W. L.; Eyermann, C. J. J. Phys.Chenr. 1982,86,4834. (e) Jolly, W. L.
J. Phys. Chem. 1983,87,26. Eland, J. H. D. " Pbtmlectron Spctroscopy"; John Wiley & Sons:Toronto, 1974, p. 132. B 6 h q M. C.; Gleiter, R. Angew. C h . ,Int. M.Engl. 1983, 22, 329. Bancrofl, G. M.; Tse, J . S . Comments Inorg. C'hem. 1986,5, 89.
(a) Jolly, W. L.; Hendrickson, D.N. J. Am. Chem. Soc. 1980, 92, 1863. (b) Agren, H.; Selander, L.; Nordgren, J.; Nordling, C.; Siegbahn, K.; Mulier, J.
Chem. Phys. 1979,37, 161.
Hubbard, J. L.; Lichterberger, D. L. J. Chem. P h p . 1981, 75, 2560. Hu, Y. F.; Bancroft, G.M.; Liu, 2.;Tan, K . H. Inorg. C h .1995,34, 37 16. Carlson, T.A " Photoelectrm and Auger Spctroscopy ",Plenum Press, New York,
22
1975.
(a) Gelius, U. "Electm Spectroscopy "; Shirley, O. A., Ed.; North Holland: Amsterdam, 1972; pp3 1 1. (b) Gelius, W.;Siegbahn, K. Furauby Disnrîs. Chem. Soc.
1972, 54, 257. (c) Bancroft, G. M.; Malmquist, P.-A.; Svensson, S.; Basilier, E.; Gelius U.; Siegbahn, K. Inorg. C h 1978, 17, 1595.
(a) Yeh, J. I.; Lindau, 1. At. Data Nucl. Data Tables, 1985, 32, 1. @) Green, J. C. Stmct. BBonng (Berlin), 1981,43, 37. (c) Cowley, A. H. Prog. horg. Chem. 1979,
26, 46.
(a) Li, X.R.; Bancroft, G.M.;Puddephatt, R.I.; Hu,Y.F.; Liu, 2.;Sutherland, D. G. J.; Tan. K. H. J. Chem. Soc.,Chem. Commzin. 1993, 67. (b) Li, X. R.;
Bancroft, G. M.; Puddephatt, R. J.; Hu, Y. F.; Liu, 2.;Tan. K. H . Inorg. Chem. 1992,31,5162. (c) Li, X.R;Bancroft,G. M.; Puddephatt, R. J.; Liu, 2.;Hu, Y. F.;
Tan, K.H. J. Am. Chem. Soc. 1994,116,9543. Hu, Y. F.; Bancroft, G. M.; Davis, H. B.; Male, J. 1.; Pomeroy, R. K.; Tse, J. S.;
Tan,K. H. Organometallics, lW6,iS, 4493.
Chapter 2 Expet-imental
2.1.
Pnpantion, Purification and Introduction of Samples
The compounds W(CO),PM%, W(CO),NBD (NBD
=
norbomadiene), cis-
W(CO),(PMe& fim-W(CO),(PMe& and~oc-W(CO),(PM%)~were prepared and purified by methods in the literature,' with some modifications. For example, a column separation
method was used for the purification of cis-W(CO),(PMeJ, rather than the sublimation method, ' because small arnounts of cis-W(CO),(PMe& could be converted to the tramisomer dwing the sublimation process. For the same reason, the temperature should be kept
as low as possible in the process of introducing the sarnple to the gas ceIl for evaporation in the photoelectron spectrometers. A culumn separation process was also used to purif) ~rans-
W(CO),(PMe& afler the cis- to tram- isomerization reaction was complete. Two eluents were used in order to separate the tram-isomer from the remaining cis-isomer and decalin
'
(the heating solvent). The latter was difncult to remove by evaporation because of the high
boiling point (>160°C). InC,H, and TIC5H5were obtained cornrnercially fiom Strem Chernicals and were
purified by vacuum sublimation.'
The extremely air sensitive and moisture sensitive
compound InC5H5was handled in a dry nitrogen atmosphere (a dry box, schlenk tube, and a vacuum line were required). The relatively stable T l C a could be handled in the air for a
short period of tirne, but it should be kept cold for future use.' The pur@of sarnples was confirmed by known methods (melting points, IR,
24
MS and Chromatography).'*' Special precautions were taken for transportation of the
sensitive samples to the remote synchrotron radiationcentre. It could be carried safely on a long journey by pachg the sarnple tubes Uedwith dry ice together in a Dewar covered with cotton wool. The needle valves of the tubes were not allowed to touch the dry ice since that might break the seal of the "0"ring.
AU the sarnples were introduced into the gas ce11 of the spectrometer directly via the heatable probe, except for InC5H5which was introduced under dry nitrogen (the operating methods for air sensitive samples were describeci in detail elsewhere. *'
' The less volatile
samples required heating in order to generate enough vapor pressure. The pressure in the sample chamber was controlled to be
- IO*'
Torr, and the pressure in the gas ceIl was
around 5 x 10" Torr. It has been show-by our scpniments that a good sample, for gas phase photoelectron
spectroscopie stuclies, should be volatile and stable (to heat and light of the light source) or easy to volatilize by heating without any decomposition. In other words, the sarnple should
have low sublimation temperature and high melting point or high decomposition temperature
in order to obtain intense and reliable photoelectron spectra. In addition, it has been found haî, the lmger the dfereence between the sublimation tempercture d the meiting point
or
the &composition temperature of the sample , the better the sumple is for gas phase photuelecrm p c t r a w p i c &es.
For example, W(CO), is an excellent compound for this
study since it can be evaponited without heating under vacuum condition. TIC& is another very good sample because it can be volatilized easily by heating without any sample decomposition (very high m.p. 300 O C ) . inC5H,is volatile under vacuum even at room
25
temperature, but special skills and equipments are required to handle this air and rnoisture sensitive compound. The sublimation temperature of jac-W(CO),(PMq), is very high (- 180 OC), therefore, it is difncult to record high quality spectra for this kind of compounds.
Compounds W(CO),PM%, irans and cis isomer of W(CO),(PMe&, and W(CO),NBD al1 show small differences between their sublimation temperatures and melting points. To get reüable spectra of these compounds the sarnple heating temperatures must be controiîed strictly below their melting points, because when the temperatures reach their melting points,
big fluctuations in the sample pressure &en occur which can lead to deformation of the spectra and ( in the worst case) even shutdown of the pumps or whole instruments. The sublimation temperatures and melting points of the organometallic compounds studied in this work are summarized in Table 2- 1. Table 2- 1. The sublimation temperatures, melting points, and references of the organometallic compounds studied in this work Compound
Sublimation temperature
Melting point
Reference
w~5G&)
40 k 10 OC
Tl(%H5)
90 10 OC
300 "C
2
w(Co),
30
10 OC
170 OC
12
w(cO)phle,
45
10 "C
56 "C
1
fr41tsœw(c0)4(PM% )2
82 O C
1
cis-W(CO),(PM% ),
IO "C 90* 10°C
108 OC
I
W(CO),NBD
80* 10°C
90 O C
1
fww(co)3(PM%)3
190A 10°C
300 "C
1
2.2
*
70
2
Recordiag the Pbotoclcetron Speetra with Heliurn Light Source
26
In this work, the He 1 and He il photoelectron spectra of the studied organornetallic compounds were obtained by using a modified McPherson ESCA-36 photoelectron spectrometer. This spectrometer has been described in detail previously.' Briefiy speaking, it has 6 major components: (1) a helium hoUow cathode discharge lamp which can generate He 1(21.22 eV) and He II (40.81 eV) resonance lines; (2) a vacuum (sample) charnber with a Edwards Senes 100 diffusion pumping system and a gas cell; (3) a hemispherical sector
electrostatic analyzer with a 36 cm radius and a 10 cm gap between the spheres. The photoelectrons with difEerent kinetic energies can be separated and analyzed when passing
through this analyzer. It is pumped by a turbomolecular pumping system; (4) a channeltron detector which rneasures the intensity of the photoelectrons with certain (or selected) kinetic energy; (5) a Zenith 2-158 PC microcornputer, which collects the electron signals Born detector and controls the kinetic energy scanning of the electron energy analyzer; and (6) a heatable sample probe through which solid sample can be evaporated and introduced to the gas cell.
Table 2-2. Working parameters for recording the PE spectra of the studied
compounds with helium light source --
-
Name -
--
-
Parameter
--
10"to~
Base pressure of the chamber (Ph)
s2
, P Pressure with helium gas (P,+ )
3.5 x 104 tom
Total pressure with sarnple (Ph + P , + P-)
- 3 x 20
Working voltage of the channeltron
2450 V
Working cument of the He lamp
300 mA
x
torr
The Ar 2p,, line at a binding energy (BE) of 15.759 eV was used as the interna1 caliiration for the spectni of shidied compounds dunng data acquisition. The typical working parameters for recording the photoelectron spectra with the helium light source are listed in Table 2-2. Under these conditions, the resolution (de6neû as the FWHM of Ar 2p, line) was
better than 25 meV for the He 1 spectra and about 36 meV for the He ïi spectra, and the intensity of Ar 2p, line was about 15,000 counts/second for He 1 spectra and about 300 countskand for He U spectra. The PE spectra were fitted by using a linear combination of Gaussian-Lorentzian line shapes with an iterative procedure described previ~usly.~ The main purposes of recording helium PE spectra of the studied compounds are as foliows: (i) The srnail natural width of the helium beam allows us to obtain high resolution He I spectra, in which not only the levels due to different electronic states can be separated, but afso the vibrational structure can be observed. For example, the splittings due to spin-
orbit coupling and ligand field effect, as well as the vibrational structures due to CO and MCO stretchings have been resolved for W 5d spectra of W(CO), and its denvatives in this
study. The observation of these fine structures is helpfùl for the correct assignment of the spectra of metal d orbitals and for the study of the interactions (or bindings) between metal and ligands. (ii) The optimized working conditions can be obtained by recording the He spectra of each compound in our laboratory. Because these conditions are the sarne as those required in the synchrotron radiation centre, this work can save a lot of precious beamtime in the synchrotron radiation centre. (iii) The He 1 spectra of the studied compounds are caiibrated i n t d y by the Ar Zp, line; therefore, these spectra can be used as the references for the studies by synchrotron radiation.
28
2.3.
Recording the Photoelcctron Spectra with Synchrotron Radiation The spectra at higher photon energies were recorded on a similar PE spectrometer
'
with the Grasshopper bearnline at the Canadian Synchrotron Radiation Facility (CSRF) which is located at the Aladdin storage ring, University of Wisconsin-Madison.' The synchrotron radiation was monochromatized by a Mark IV Grasshopper monochromator which provides light with energy ranging from 22 to 500 eV. The 600 groovehm grating
and the 1800 groovdmm grating were u s d in this monochromator, respectively, to offer photons with energies ranging fiom 20 to 75 eV and photons of energy from 70 to 200 eV. Inside the Grasshopper b d i n e , the light is first focused by a mirror and then passes the
entrance slit. Mer that it strikes on the grating, which rnonochromatizes the light. Finally the light bearn passes the exit slit and enters the gas chamber. The wavelength of the monochromatized üght can be chosen by changing the position of the mirror and grating. The photon resolution (4of the rnonochromatized light depends on three parameters, the spacing of the ruling on the grating (associated with x parameter), the adjustable widths of the dits (w,pm), and the selected photon energy (E, eV), based on
AE= E2aÂ/12398
(2.1)
where al = nu and x = 0.008 for the 600 groove/mm grating, and 0.0027 for the 1800 groove/mrn grat A Quantar (Mode1 3395 A) position sensitive detector (PSD) has been used since 1991 together with the ESCA 36 photoelectron spectrometer to enhance the intensity of the
signal and minimize the experimental time.l0 The operation of the CSRF spectrometer was similar to that of our laboratory spectrometer with the helium light source. The working
29
conditions in CSRF for the studied samples were the same as those in our lab with helium light . The spectra were caiibrated using Xe gas and the calibrated He 1 spectra of the samples. The recent reported spectra, such as the spectra of W(CO),
which has been
calibrated, can also be used as the references for spectral calibration. Al1 spectra were deconvoluted with a Gaussian-hrentzian iine shape using a noniinear least-squares procedure described e1sewheree6The peak (or band) areas were used to calculate the experimental branching ratios (BR,) or relative intensities for each peak (or band), based on the simple
formula, BR, = A, /CA, where A, is the individual peak area. The cornparison of the experimental BR, with the thearetical BR, calculatecl by Xa rnethod or Gelius mode1 can assist the spectral a~signrnents.~ l2
Since the photon energies can be changed continuously over a wide range with the synchrotron radiation source, variable energy photoelectron spectra can be obtained with
great convenience In this work, rnany high resolution broad s«ui and narrow valence spectra of the studied samples were recorded under different photon energies, which demonstrated the great power of synchrotron radiation in photoelectron spectroscopy.
30
2.4.
Refcrencts
(1)
(a) Strohmeier, W. Angew. C h , lit. Ed Engl. 1964, 3, 730-737.
(b)
Darensbourg, M. Y.; Conder, H. L.; Darensbourg, D. L.; H d a y , C. J. AmChem.
Soc. 1973,95, 5919. (c) Mathieu, R.;Lenzi, M.; Poilblanc, R. Inorg. C'Che. 1970, 9, 2030. (d) King, R. B.; Fronzagiia, A. Inorg. Chem. l966,S, 1837- 1846. (e)
King, R. B.;Raghu Veer, K. S. Inorg. Chem. 1984,23, 2482. ( f ) Jenkins, I. M.;
Verkaâe, J. G. h r g . Chem. 1967,6,2250. (g) Jenkins J. M.; Moss, J. R.; Shaw, B. L. J. Chem. Soc.(A), 1969, 2796. (h) Bancroft, G. M.; Dignard-Bailey, L.;
Puddephutt, R. J. Inorg. C h . 1984, 23, 2369.
(2)
(a) Fischer, E. O. Angew. Chem. 1957,69, 207. (b) Meister, H. Angew. Chem. 1957,69, 533. (c) Cotton, F. A.; Reynolds, L.T. J. Am. Chem. Soc. 1958,80,269.
(d) Nielson, A. J.; Rickard, C. E. F.; Smith, J. M. Inorg. S ' t h . 1986, 24, 97. (e) Fischer, E.O.; Hofinam, H.P. Angew. Chem., 1957, 69,639.
(0Poland, J. S.; Tuck,
D.G.J Olgmmet. Chem., 1972,12,307. (g) Peppe, C.; Tuck, D. G.; Victoriano,L. J. Chem. Soc., Dalion, Truns. 1981, 2592. (h) Lalancette, J. M.; Lachance, A.
Gan J. Chm. 1971,19,2996. (i) Shibata, S.; Bartell, L. S.; Gaviq Jr. R.M. J Chem. Phys. 1964, 41, 717. (j) Egdell, R G.; Fragaia, 1.; Orchard, A. F. 1 Electton Spctrosc. relut. phenom. 19711,14,467. (3)
Li, X. "Photoelectron Spctroscopy of Organometaliic Compoundr", Ph.D. Dissertation, The University of Western Ontario,London, Ontario, Canada, 1995.
(4)
Shriver, D.F.;Drezdzon, M.A "meMmîpla~ionof Air-Semîtive Compounds ",
Second Edition, A Wrley-Interscience Publication, by John Wiley & Sons, hc.1986.
31
(5)
(a) Coatsworth, L. L.; Bancroft, G. M.; Creber, D.K.;Lazier, R. J. D.; Jacobs, P.
W. P. J Elect. S ' c t . Reiat. Phenom. 1978, 13, 395. (b) Dignard, L. M. Ph.D. Dissertation, The University of Western Ontario, London, Ontario,Canada, 1986.
(c)Yang, D. S. Ph D. Dissertation, The University of Westem Ontario, London,
Ontario,Canada, 1989. (6)
Bancroft, G.M.;Adams, 1.; Coatsworth, L. L.; Bemewitz, C. D.; Brown, J. D.; Westwood, W. D. Aml. Chem. 1975,47, 586.
(7)
(a) Bozek, J. D.; Cutler, J. N.; Bancroft, G. M.; Coatsworth, L.L.;Tan, K.H.;
Yang, D. S.; Cavell, R. G. C'hem. Phys. Lett. 1990, 165, 1, @) Cutler, J. N.; Bancroft,G. M.; Bozek, J. D.;Tan,K. H.; Schrobilgen, G. .I. Am. Chem. Suc. 1991, 113, 9125. (c) Cutler, J. N.; Bancroft, G. M.; Tan, K. H. Chem. Phys. 1994, 181,
461. (d) Sutherland, D. G. J.; Bancrofl, G.M.;Tan, K.H. J. Chem. Phys. 1992, 97,
7918. (8)
Tan,K.H.; Bancrofk,G.M.; Coatsworth,L.L.; Yates,B.W. C'and Phys. 1982, 60, 131.
(9)
Cutler, J . N. PhD. Dissertation, The University of Western Ontario, London, Ontario, Canada, 1992.
(10)
Liu, 2.F.; Bancroft, G. M.; Coatsworth, L. L.; Tan, K. H. Chem. Phys. htf.1993, 203,337.
(1 1)
(a)
Hu,Y.F.; Bancroft, G.M; Bozk, J. D.; Liu, 2.F.; Sutherland, D. G. J.; Tan,
K.H. J. Chem. Suc, Chem. Commun. 1992,12764278. (b) Hu,Y.F.; Bancroft. G. M.;Liu, 2.;Tan, K.H. Inorg. Chem. 1995,34,37 16-3723.
32 (1 2)
Hu,Y.F. Ph D.Dissewon, The University of Western Ontario, London, Ontario, Canada, 1996.
Chapter 3
Photoelectron Spectra of Trimethylphosphine Substituted Tungsten
Carbonyls
3.1.
Introduction
Photoelectron specîroscopy (PES)has proven to be a valuable tool for the study of the electronic structure of transition-metal complexes since the first studies of Ni(CO),, Fe(CO),, and Mn(CO)5X(X = halogen, etc.) in 1969.' Possibly, because of their synergic bonding properties, many PES studies have been focused on transition-metal carbonyls and
their denvatives, with the aim of studying their electronic stnictures. In particular, the
M(CO)& complexes (M= Cr, Mo, and W; L = substituted ligand, such as phosphine, etc., and n = 0, 1,2, 3) have amacted considerable attention2 Two principles (or models) related to the ligand electronic effects on a transition-rnetal center have been found usefiil in the
photoelectron spectroscopic studies of these complexes. The first one, the ligand additivity model proposed by Bursten in 1982,' States that valence metal orbital ionizations are systernatidy and repramicibly shiAed by an amount directly proportional to the nurnber and type of ligand substitutions on the metal center. This model was proved to work well in Bursten's electrochemical studies of valence metal d orbitals in M(CO),(CNR),,
complexes,'
and in valence photoelectron spectroscopic studies of MO(CO),(PR,),~"
W(CO),(PR,),~.
and
A similar principle was found to be true in Feltharn and Brant's X-ray
photoelectron spectroscopic WS) studies of core ionkition shifts of solid ~omplexes.~ The
34
second principle States that, when compating related molecules, the binding energy shift of
a nonbonding valence orbital localized on a particular atom of these molecules should be eight-tenths of that particular atom's core binding energy shift between two molecules, i.e.
di
-,
. This me-valence ionization correlation pnnciple was descnbed by
= 0.8 ,AE , (
Jolly and was applied to understand the valence spectra of main-group molecules.' This principle was also found valid for Fe(CO),L complexes by Joliy Mo(CO),(PM%),
and for the
series by Lichtenberger and co-workers.*
Previous studies * have show that the binding energy shifis in metal orbitals with
-
ligand substitutionsdepend on the total donor ability (a-donation n-acceptance) or donor acceptor ratio of the substituted ligand relative to that of parent ligand; while the ligand field splittings in the metal orbitals of low spin d6 octahedral complexes depend only on the relative x-accepting ability of the substituted ligand and the parent ligand. For example, ligand field spüttuigs of the $ orbitals and binding energy shifts toward lower energy for the metal d orbitals have been observed in the spectra of MQ(CO),(PR,),~
- " %*
and
W(CO)~(PR&~"-*when CO is replaced by PR, which is a stronger a donor but a weaker
n acceptor than CO. The spinsrbit coupling theory was first used by Hall to interpret photoelectron spectra of transition metai systerns. In particular, the large spin-orbit splitting in the 5d orbitals of the third row transition metal complexes has been usefùl to obtain a definitive
assignrnent for the spectm of XRe(CO), species? Based on the sarne theory, the spin-orbit coupling constants (0 and ligand field splittings (e-bJ were obtained in the studies of
M(CO)&= (M = Cr, Mo, and W, L = PM% PEt, P(NMeh P(OMe),, P(OEt),, and PF,) and
w(co),(PR3)n.h Recently, the high resolution broad-scan photoelectron spectmrn of W(CO), was obtained in our group using synchrotron Tadiation. For the first time, this spectrum covered
the valence, h e r valence, and core level regions with high resolution. The vibrational structures in W 4f core level spectrum were aiso observed.' In this chapter, high resolution broad-scan gas phase photoelectron spectra (which cover valence, imer valence, and core levels) are reported for a series of trimethylphosphine substitut4 tungsten carbonyls. The
inner valence and core levd spectra can be interpreted and assigned based on cornparison with published results. Better resolution has been achieved in the newly obtained He 1 spectra of
the valence level and W 5d regions of these complexes (compareci with the previous spectra). Spin-orbit splittings, ligand field splittings, and vibrational structures are observed in the spectra of both W 5d and W 4f regions. The phosphorus 2p spin-orbit components of the phosphine substituted complexes have been resolved for the first tirne. With ligand substitution, al1 the metal and ligand orbitals shifi with different degrees to lower energy, because phosphine is a stronger a donor and weaker ir acceptor than CO. Linear binding energy shift trends are found in both core and valence levels of metal and phosphorus ionizations, which confirm the ligand additivity predictions for these complexes. The corevalence ionization correlation principle can be illustrated by comparing the binding energy
shift data between cote and valence levels. 3.2.
Experimentai Section
The compounds W(CO)5PM%, W(CO),NBD, ch-W(CO),(PM+),
&ms-
W(CO),(PMe& and fac-W(CO),(PMeJ, were prepared by methods in the literature,' with
36 some modifications. For example, a colurnn separation method was used for the purification ofcis-W(CO),(PMe& rather than the sublimation method,' because small amounts of cjs-W(CO),(PMe& muid be converted to the hm-isomer during the sublimation process. For the same reeson, the temperature should be kept as low as possible in the process of introducuig the sarnple to the gas ceIl for evaporation in the photoelectron spectrometers. A column separation process was also used to puri@ frans-W(CO),(PM%), &er the cis- to tram- isomhtion reaction was complete.
Two eluents were used in order to separate the
banoisorner &om the remaining cis-isomer and decalin (the heating solvent). The latter was dificult to remove by evaporation8because of the high boiling point (>160°C).
AU the sarnples were introduced into the gas ce11 of the spectrometer directly via the heatable probe. The less volatile sarnples required heating in order to generate enough vapor pressure. Temperatures for vaporization of these samples were as follows: W(CO), (35
* 5 T),W(CO)pMe, (50 + 5 OC), cis-W(CO),(PMe,), (65
5 OC),fac-W(CO)3(PMq)3 (180
(90
5 OC), pans-W(CO),(PMe&
5 O C ) , and W(CO),NBD (75 + 5 OC). The pressure
in the sample chamber was cuntrolled to be around 4 x 10" Torr, and the pressure in the gas ceIl was around 5 x lO" Torr. The photoelectron spectra were obtained by using two different photoelectron spectrometers. He 1 spectra of sarnples were recorded using a McPherson ESCA-36 photoelectron spe~trometer.~ The Ar 2p, üne at a binding energy (BE) of 15.759 eV was used as intemal celibration dunng data acquisition. The spectra at higher photon energies were recorded on the modified ESCA-36 spectrometer with the Grasshopper beamlllie 'O at
the Canadian Synchrotron Radiation Facility (CSRF) which is located at the Aladdin storage
37
ring, University of Wisconsin-Madison." A Quantar Mode1 3395 A position sensitive detector (PSD) was used together with the ESCA 36 photoelectron spectrometer to enhance
the imensity of the signal and m h h h the acpnimentai tirne.'* The spectra et higher photon energies were calibrated using the Xe 5s line at a BE of 23.397 eV and the calibrated He 1 spectra of the samples. The spectra of W(COl6 which have been calibrated and pubüshed' can also be used as the intenial caiibration data. Spectra were deconvoluted wit h a GaussianLorentzian line shape using a nonlinear least-squares procedure described elsewhere." 3.3.
Resulb and Discussion
3,3.1. Central Features
The high resolution broad-scan photoelectron spectra of W(CO),, W(CO),PM%, cis-
W(CO),(PMe& trm-W(CO)4(PMe&,, jac-W(CO),(PMeJ,,
and W(CO)4NBD at 80 eV
photon energy are presented in Figure 3-1. One of the important points of recording these broad-scan spectra is that, for each cornplex, ail features can be seen immediately in one specûum: the relatively intense and narrow valence bands with Eb less than 20 eV; the weak,
-
broad inner valence bands with Eb between 20 40 eV (from S to D); and both very narrow
core levels (C, and CJ and a weak, broad core level (C,) with Eb around 40 eV. Another important point for obtaining these broad-scan spema is that the difference and sirnilarity of these complexes in the whole spectral range can be observed clearly by cornparison, which,
in tum, can assist us to interpret and assign these spectra based on our recently reported
-
results of W(C0): (the spectnim was recordeci again and is shown in Figure 3 1(a)). Al1 the phosphine substituted tungsten wbnyls have similar featues in their spectra, except for their different shifis in E, and the bigger spîitting of W Sd and phosphorus 'lone pair' bonding
38
orbital in the tram-isomer. However, they are dinerent h m the starting material W(CO& in that they have an extra band P around E, 10 eV which has been assigned previously as the
". difference is observed in the phosphorus 'lone pair' bonding orbital or ~ ( w - P ) ~ ~Another inner valence level: the intensity of band S decreases with the increase of PM% ligmd substitution (or with the decrease of the number of CO); however, the intensity of band T
increases with the increase of ligand substitution. This trend clearly ülustrates that band S is related to the CO ligand and band T to the PM% ligand. In fact, band T can be assigned
to an orbital containing mainly C 2s character of the substituted groups according to the published results.14 A sirnilar trend was also found in the spectra of Os(CO), and Os(CO),PM%, but with a different assignment.lS We think that our new assipunent and interpretation for the i ~ evalence r levels of phosphine complexes are more reasonable than the previous one" because they are based on the systematic study of the ligand substitutions. The intense and very narrow peak between band S and band T, only observed in thefaccomplex of this study, possibly results from the sarnple decomposition at bigh temperature (185 OC). Since di other bands are similar to those of W(CO),, they can be interpreted and
assigned according to w published resuits of w(C0);. Table 3-1 lists the positions, widths and assignments of bands in the i ~ evalence r and core level regions. For the valence level, band 3 arises from three MO'S (6t,,4eP and 7a,, orbitals) of mainly CO 4 0 character. ûther valence bands are shown in Figure 3-2. Previous studies of these complexes onfy reported the results of the outer valence or W 5d region,"
- " and the inner valence and core level
bands of the phosphine substituted complexes have not been observed before. In addition, Our newly obtained valence level He 1 spectra for W(CO),PMq, cis-W(CO),(PMe&, and
39
I~~-W(CO),(PM&(Figure 3-2) have better resolution than the previous ones." In these spectra, the band assignments are show on the figure. There are two band regions which are
worthy of special attention. First, the bands in the region around 10 eV (shown as P in Figure 3- 1) are due to ionizations âom the preûominantely o(W-P) orbitals. The cas-isomer shows only one ionization band in this region, which is broader and more intense than the similar band in the monophosphine cornplex. The intensity of this band in the cis-isomer
indicaies that the ionizations of the two o(W-P) orbitals are essentially degenerate. A large splitting (1.46 eV) can be observed in the ionkations of the two o(W-P) orbitals from the ttm-isomer, and the two bands are well resolved. In thefac-cornplex, these ionkations are again close (only one band can be seen, see Figure 3-1). These experimental results are very similar to those obtained by Lichtenberger and co-workers in their study of
Mo(CO),(PMe3,
complexes,* and therefore, the large splitting of the a(W-P) band in the
truns-complex can be arplained similady as due to the energy differences between the d, and pzstabilizations.* Second, four components can be resolved from the W 5d band in almost al1 these spectra (especiaily for tram-isomer), which are due to spin-orbit coupling, ligand field spütting and CO vibrational splitting. These wül be dimssed in detail in the next section using our newly obtained high resolution W Sd spectra. The bands in the region with E, greater than 11 eV in Figure 3-2 derive prllnarily from the Su and l x orbitals of the CO ligands. Also observed in this region are bands due to o(C-H) and a(P-C) for phosphine complexes. This region is & d t to interpret in detail for organometallic complexes because of the large number of overlapping ionization bands," and we do not attempt to give a
detaiied interpretation.
4 (b) W(CO),PMe,
1
Figure 3- l . Broad-scan PE spectra of (a) W(CO),, (b) W(CO)5PM%,(c) cis-W(CO),(PM% h, ( d ) r i ~ r i ~W(CO),(PMe3)2, s(e)fac- W(CO)3(PMel))oand (f ) W(CO),NBD.
Table 3-1. Binding energies &),widths (WJ, and assignments of the inner-valence and core level spectra of W(CO),(PMe&
-
(n = 1 3)
A
24.01
2.12
23.60
2.13
23.18
2.13
23 .O7
2.14
satellite 1
B
26.92
2.3 1
26.14
2.33
25.96
2.30
25.55
2.23
satellite 2
C
30.3 1
2.74
29.2 1
2.75
29.08
2.75
28.44
2.80
satellite 3
3.3.2.
Valence Ltvel W Sd and Corc Levtl W 4f A High resolution close-up of W Sd spectra.
on the tungsten 5d orbitais in W(CO),(PR,),
Our previous photoelectron stud?
complexes has confirmed experimentally the
general vaiidity of the ligand additivity principle in that (i) the experimental ratio of the W
bsplitting (or ligand field splitthg) for W(CO)SR, cis-W(CO),(PR,), trans-W(CO),(PR,),, andfac-W(CO&(PR& is in qualitative agreement with the theoretical predictions (1 :-l:2:0);
(i) a plot of the first ionization potential (iP) or E, for the W 5d levels vs. n (number of ligand substitution) shows a good linear correlation and the first IP's of cis and
isomers are
very sVnilar as prediaed. In figure 1 of the previous studp (which showed the spectra of W 5d region for some of the substituted W(CO), species), ody a doublet of intensity
- 2: 1 due
to the spin-orbit splitting of the ta orbital was seen for W(CO), and fac-W(CO),(PMeJ,; however, three peaks were observed for cis and truns complexes due to the ligand field splitting of the ta level into bt and e,
(trats) or b,
and e (cis) and the spin-orbit splitting of
the e (or eJ MO. Because PR,(such as PMQ is a poorer sr acceptor than CO, the b, (or b S MO has a larger IP than the e (or eJ MO in W(CO)QR, and ~~~s-W(CO),(PR,)~, but with
the Opposite order in cis-W(CO),(PR&.
Our newly obtained spectra of W 5d region for W(CO)5PM%,cis-W(CO),(PMq), and &m-W(CO),(PMeJ2 (see Figure 3-3) show better resolution than the previous ones.&
In addition to the components of spin-orbit splitting (the splitting between band 1 and band 2) and ligand field splitting (the splitting between band 3 and the average of band 1
+ banà 2), another component (the high energy shoulder) which is due to the vibrational fine structure of CO is aiso observed very clearly in these new speara, such as 2' and 3' bands.
44
Additionai vibrational structure fiom W-Cvibration ( -50 meV) has been resolved in the spectrum of W(CO)6.74l6 This vibrational structure broadens the spectra in Figure 3-3, but c m not be fitted readily. The C-O vibrational splitting has been reported for the valence spectra of w(cO),,"'~
CpM(CO), (M= Re and Mn)17 and Mo(CO),(PR,)/'
AU the bands are fitted with the same width for a h spectrum. Because of the larger ligand field splittings in the spectra of iras-W(CO),(PMq), and W(CO),NBD, another CO
vibrational band has to be fitted in order to obtain a reasonable fit for both of them. It is noteworthy that our new spectrum of cis-W(CO),(PMq), is different from the old one," which contained a minor component due to trm-isomer (the product was purified by
sublimation at high temperature in the previous wo*"
which evidently causes a small amount
of cis-isomer to be converted to the trm-isomer). The fitting parameters of W 5d spectra are listed in Table 3-2. The spin-orbit coupling constants
(c) and ligand field splinings (A )
are obtained based on spin-orbit coupling the or^,^ which are in rather good agreement with out previous resuitsa and theoretical predictions for this series of phosphine complexes (see
Appendk B.2) The ligand field splitting (A) in the W Sd region increases in the order of < W(CO)QMe, < cis-W(C0),(PMeJ2 < tram-W(CO),(PMe,), w(co)6=f~-w((coX(PM~ < W(CO)JWD, which lads to the increase in width of the whole W Sd spectral envelope in
the sarne order. However, the tungsten spin-orbit coupling parameters are almost constant for W(CO), and the phosphine substituted complexes. A diagram showing the synergic bonding in W(CO),(PM&
is given in AppendDr B. 1. A cornparison of ligand field splittings
in the isomers of W(CO),(PMq),
is included in Appendk B.2.
The spectrum of W(CO),NBD wiU be disaisseci in a separate section.
Counts
Table 3-2.Band positions (eV), widths (eV), assignments, spin-orbit coupling constants (O, ligand field splittings (A = 4 e or b,, e,), average binding energies (eV), and their shifts (eV)relative to W(CO), in W 5d spectra of the listed complexes.
-
W(cOk 1 2
2'
W(~OhPM% 1 2
3
3' nans-W(CO),(F'Mc,h
I 2
2'
3 3' cis-W(CO),(PMC,~
3 1
2 2'
foc-W(COh(PMG 1 2
W(COpJE3D 3 3' 1 2
2'
-
47
IL High resolution spectra of corn level W 4J In the past, core level spectra of inorganic and organornetauiccomplexes were recorded by using XPS with low resolution and these spectra could only be useâ to study chernical shift effkcts. Recently, however, with high
resolution synchrotron radiation, it has been possible to resolve vibrationai and ligand field sphttings on the core p and d levels of inorganic m~lecules'~~ l8 and on the metal 4f levels of organometallic complexes.'p
''
Studying high resolution core level spectra of a group of
phosphine substituted tungsten carbonyls is important because we wanted to know not on1y the effect of chernical shiAs but also the infiuence on the width of the core level spectra by
ligand replacement. Figure 3-4 shows the high resolution spectra of W 4f levels for W(CO), and its phosphine substituted complexes. Spectnun (a) has been published recently by our group.' This spectrum shows mainly two spin-orbit components W 4 f , and W 4fjn with binding energy at 37.98 eV and 40.16 eV, respectively. The CO vibrational structure has been resolved for the first time in the hi& energy shoulder of the bands.' Spectnim (b) in Figure 3-4 is obtained by mixing a smdl amount of starting material W(CO), with the W(CO)QMe, sample and recording the spectra at different temperatures. Since the binding magies of the two bands fiom W(COI6 are known (spectrum (a)), the binding energy shift
of the core level W 4f bands caused by ligand substitution can be seen directly and immediately f?om spectrum @) (in which band 1 and 3 belong to W(CO),PMe, and band 2
and 4 to W(CO),J. In addition, the two bands of W(CO), can be used as intemal calibration of the bands that belong to its monophosphine derivative. In Figure 3-4@-e), the high resolution W 4f core level spectra of a senes of phosphine substitut& tungsten carbonyls are reported for the first the. AU these spectra
48
show botb the two strong bands of W 4f spin orbi components and the smaU shoulders of CO vibraiionai fine structure. In order to compare the influence of ligand replacement on the W 4f spectra without involving the effect of variation in photon energies, spectra @)
- (f) in
Figure 3-4 are ail recorded at the same photon energy (102 eV). The experimental results and fitting parameters for the binding energies (E,), shifts, and widths of W 4f bands of the studied complexes are iisted in Table 3-3. A greater binding energy shiR can be seen obviously from these results with ligand substitutions and an almost linear correlation is established between the core binding energy shift and the number of ligand substitutions. In addition, the width of W 4f bands increases slightly from W(CO), to nrms-W(CO),(PMe,J,
following the order: W(CO), (0.30) = fac-W(CO),(PM&
4
W(CO),PMe, (0.3 1) + cis-
W(CO),(PMe& (0.32) 4 rrmt-W(CO),(PMeJ, (0.33) + W(CO),NBD (0.35). The trend of increase in the width of W 4f bands is similar to the trend of ligand field splitting observed in the W Sd spectra and therefore can be explained as due to ligand field effects on the core 4f
orbitals. A diagram showing the correlation betwmthe ligand field splitting of W 5d spectra and the width of W 4f bands is given in Figure 3-5 (which has a regression coefficient of
? = 0.9065). An even larger ligand field broadening has been seen recently on the Os 4f
"
levels of complexes Os(CO),L (L = CO and PM%).
"
Figure 3-4. High resolution W -If core level sprctra of ( a ) W(CO),. (b) W(CO), + \\.'(CO),PiCle,. ( c ) cis-W(CO),(Ph.lrj h. (d) rram-W(CO1(PM%h . (e)foc-W(COh (PM%h . and ( t) W(CO),NBD.
50
Table 3-3. Fitting parameters' of W 4f spectra of the listed complexes
cornplex
binding energy (eV) 4f, 4% average
W(cO)6
37.94
40.10
39.02
spin - orbit splitting (eV) 2.165
shift (eV)
width (eV)
0.00
0.30
'error for binding energy: 0.02 eV; for spin - orbit splitting:i 0.005 eV; and for width: 0.01 eV.
0.29
0.30
0.31
0.33
0.33
0.34
0.35
0.36
Half Widtli (eV) of W -Cf Figure 3-5. A diagram showing the correlation between the ligand field splittin; of'W 5d bands and the width of W 4f bands.
W5d
ci
W-If A
P lone pair
V
P 2p
O
1
2
3
4
Number of Pliosphine Ligands ( n ) Figure 3-6. Shi ft comparison diagrmi for tungsten and p liosplioriis binding energy
shifis. W 5d (valence), W I f (core). P lone pair (valence). and P I p (core).
-3
-2
-1
O
Core Level Binding Energy SliiR (eV)
Figure 3-7. Core - valence shifi correlation for tungsten and phosphorus ionizations. W 5d - W 4f+, and P lone pair - P 2p.
54
C Sliajt compm'sons and core-valence ionizaîion correlaîionr. Bursten' s ligand additivity model was onginally designed for valence metd ionizations. However, it can also be used directiy for core level ionizations as shown in our above expenmental data and other
published r e ~ u l t s .Compared ~'~~ with valence ionizations, the overlap and hyperconjugative interactions between metal m e orbitals and ligand valence orbitals are much smaller than that
between metal valence orbitals and ligand valence orbitals? a Therefore, a greater ionization (or binding energy) shifi in core level than in valence level is expected when the change in
charge potential on the metal center (due to ligand replacement) occurs. According to Jolly's core-valence ionization correlation model,' when comparing the spectra of two related molecules, the binding energy shift of a valence orbital localued on a particular atom of the molecules should be eight-tenthsthe core binding energy shi. of that particular atom between the two moldes, i.e. AE-,
= 0.8
Our experimental data on binding energies and
their shifts relative to the starting matenal, W(CO),, in both W 5d and W 4f levels of these phosphine complexes are listed in Table 3-2 and Table 3-3, respectively. A graphical presentation of the core and valence data is shown in the shifl cornparison diagram of Figure 3-6. The abscissa of the diagram is the number of phosphines in the cornplex and the ordinate is the shifl in electron volts(eV) relative to the starting material, W(CO),. The valence metal shifts shown in Figure 3-6 and Table 3-2 are obtained by comparing the average
W 5d binding energy values of these complexes with that of W(CO),. The core shifts for both W 4 f , and W 4f, components are the sarne, and therefore, either their average binding energy values or the binding energies of any one of the spin orbit components can be used to
compare the shifts. As illustrated in Figure 3-6 and Table 3-2
- Table 3-3, the binding
55
energies (or ionization potentials) for both valence W 5d and core W 4f levels are shified almost linearly toward lower energy regions with each successive ligand substitution. The
-
shidt per phosphine substitution is 0.66 k 0.03 eV for the W 5d ionization (using the average
value of the binding energies of al1 fined W Sd peaks except for the peak of vibrational
-
shoulder), and 0.76 0.03 eV for W 4f ionkations ( for both W 4 f , and W 4 f d . These
data confirm the validity of ligand additivity predictions for both valence and core level shifts in these complexes. The core-valence ionization correlation can be seen immediately for these complexes when the W 5d shifi is plotted against the W 4f shifi (Figure 3-7). The ratio
*
of the valence metal d level shifts to the core metal shifts is 0.86 0.03 (Le. , ,& (
/AE(-,
* 0.03).
= 0.86
3.3.3. Eigher Energy Spectn and Phosphorus 2p Bands
The most important advantage of using synchrotron radiation is that the photon energy of the light source can be changed continuously within a wide range. Thus, relative partial photoionization cross sections as a function of photon energy can be exarnined in detail. This has proven to be an invaluable assignment tool in photoelectron spectroscopy and has provided crucial information about fundamental photoionization processes because photoionkation cross sections for different atomic and molecular orbitals Vary greatly with photon energy (due, for exarnple, to shape resonances, pronounced maxima and minima, Cooper minima effects and etc.).19*
For M(C% (where M = Cr, Mo, and W) complexes,
the energy dependence of cross sections has been studied by Green and CO-workêrs.'"
Similar variation trends in cross sections of valence W 5d and core W 4f bands are also found
in Our studies for phosphine substituted tungsten complexes. For example, with photon
56 energy increase from 80 eV to about 100 eV, the relative intensities of valence W 5d bands
decrease slowly and that of core W 4f bands increase greatly to almost their maxima. These variations can be seen obviously by cornpuhg the broad-scan spectra obtained at 80 eV (Figure 3-1) and 100 eV (Figure 3-8 (a)). In addition to these features, new bands on the low energy side of the W 4f bands (shown as 2p in Figure 3-8 (a), for example) are observed in ail 100 eV spectra of the phosphine substituted complexes (we oniy show the spectra of cis-
W(CO),(PM& ). Baseû on our experimental data, it is clear that these bands are related to
the phosphine ligands and the photon energy of 100 eV, because (i) they are found only in the spectra of phosphine complexes, not in those of W(CO), and W(CO),NBD; (ii) they are found only in the spectra recorded at about 100 eV for phosphine complexes, not in the spectra at 80 eV and 90 eV; (i) the purity of samples has been proved by the data from other techniques, such as NMR, melting point measurement and chromatography. By carefùlly studying the position of these bands and comparing them with the reference binding
energy values for phosphorus 2p, these bands can be assigned to the second order ionizations (at
- 200 eV) of phosphorus 2p,
and 2p, orbitals. This assignrnent is confirmed by carefùlly
calculating the kinetic energies of these two bands, then their binding energies for second
order ionization, and by comparing these values with that obtained directly fiom higher energy first order spectra of the 2p level, such as the spectrum of cis-W(CO),(PMeJ, obtained at 152 eV photon energy in Figure 3-8 (b). Since binding energy of electrons in a certain orbitai does not change with photon energy, only kinetic energy changes with photon
energy. When the photon energy changes from 101 eV to 102 eV, the positions (or binding energies) of W 4f bands for cis-W(CO),(PMe,), (Figures 3-8(c) and 3-8(d)) do not change;
57
but the positions of the P 2p bands shifi relatively by 1 eV toward lower E, (higher Ed, because the second order photon energy increases by 2 eV (fiom 202 eV to 204 eV). This
experimental evidence further confhns our assignrnent of the two bands. The widths of the phosphorus 2p bands are broadened by vibrational splittings as for Our previously published Si 2p spectra of si(CH,),.'"
Table 3-4 gives the binding energies (E,) and widths of the
phosphorus 2p bands in W(CO),(PMe&,
together with the mean binding energy of
the o(W-P) orbital. These data show that there is an initial large increase in phosphine
bhding energies when the Gsst phosphine bonds to the metal because the metal accepts some
-
of the electrons of the ligand. The P 'lone pair' is stablized by 1.51 (10.09 8.587 eV,and
-
the P 2 p , orbital by 0.48 (136.58 136.107 eV. The additional stabilization of the P 'lone
pair' is mainly due to the strong bonding interaction with the metal center. With fiirther substitutions d e r the monophosphine cornplex, the phosphine levels show a destabilization trend which is additive like that in the metal levels with shifts of phosphorus valence 'lone pair' or o(W-P), and
- 0.62
- 0.46 k 0.02 eV for the
0.06 eV for core level phosphorous
2p,and 2 p , Also like that in the metal levels, another evidence of additivity is that cis and tram isomers of W(CO),(PM&
have aimost identical phosphorus ionizations (although the
'lone pair' binding energies for the cis and trms isomers of W(CO),(PMe& are quite different, the averages of the two 'lone pair' binding energies for each cornplex are very similar). The shift cornparison of phosphorus 'lone pair' and 2p ionizations of the studied
phosphine complexes is iliustrated graphicaliy in Figure 3-6,and their correlation is shown in Figure 3-7. The ratio of the 'lone pair' shifis to the 2p shifts is 0.73 0.04. SUnilar additive
shifts were also found in the study of MO(CO),(PM~&,.~
Table 3-4.
Phosphorus 'lone pair' or o(W-P) and phosphorus 2p ionizations in
W(CO),(PM%)" --
-
-
~(w-p)
cornplex
phosphorus 2p,,
mean E, (eV) E, (eV)
W(CO)$'M%
phosphorus 2p,
width (eV) E, (eV)
width (eV)
10.09
137.43(5) 0.47(3)
136.58(5) 0.48(3)
c~s-W(CO)~(PM&
9.62
136.76(8) OSO(3)
135.90(8) OSO(3)
fim-w(co),(PM%)*
9.64
136.78(5) 0.5l(3)
135.93(5) 0.52(3)
fac-w(cw3(PM%)3
9.18
136.19(5) 0.49(3)
135.34(5)
OSO(3)
60 3.3.4. Higb Rcsolution Photoelectron Spcetn of W(CO),NBD
The valence level UPS spectra of norbornadiene (NBD) and similar organic compounds were published many years ago by Heilbromer and co-~orkers,~'the photoelectron spectroswpic study of Me,PtNBD cornplex was &ed
out previously by our
group.= In this chapter, the high resolution spectra of W(CO),NBD are reported for the first
time and are shown in Figure 3-l(f), 3-2(d), 3-3(d), and 3-4(f), where Figure 3-l(f) is the broad-scan spectnim at 80 eV photon energy; Figure 3-2(d) is He 1 valence level spectnirn; Figure 3-3(d) is W 5d region close-up spectrum; and Figure 3 4 0 is W 4f level spectrum
recurded at 102 eV. These spectra differ fiom those of other tungsten complexes as follows: (i) in the spectra of W(CO),NBD, the band at around 10 eV (shown as x band in Figure 3 I(f)) results from the two
R
-
bonding orbitals of norbornadiene ligand. In the valence
spectnirn of norb~rnadiene,~' t hese two x orbitals gave two peaks at 8.69 eV and 9.55 eV, and
SQ
they shift about 1 eV to high energy when coordinated to tungsten. (ii) Band 2
contains not only Sa and lx components of CO ligands, but also o(C-H) and a(C-C) components of norbomadiene. (iii) The spectra in both W 5d and W 4f regions show much larger ligand field splittings than other complexes reported in this paper. The very weak n-acceptor ability of norbomadiene is largely responsible for the large splitting. In addition, the bhding energy shift in the spectra of W(CO),NBD is smaller than that of other complexes
because the total donor abiiity (odonation - x-acceptance)of norbomadiene is siightly larger than CO but smalla than phosphines. Thedore, the a-donor ability of M3D is weaker than that of CO, and much weaker than that of PM%. Since other bands are sirnilar to those of
phosphine substinients, they can be assigned siilarly.
61 3.4.
Conclusions High resolution photoelectron spectra of W(CO)&W(CO)QMe, cis-W(CO),(PMe&,
hm-W(CO),(PMqX, fac-W(CO),(PMe&, and W(CO),NBD have been reponed. The
advantages of monochromatized synchrotron radiation (SR) for studying the electronic structure of organometallic complexes have been demonstrated further in this paper: we can study ail the levels from valence to imer-valence and core levels with high resolution in one
specûum for each of these complexes. The high resolution and high intensity of SR is critical for the study of the inner-valence and core level spectra. The inner-vdence spectra of the substituted tungsten complexes are similar to that of W(CO), which is dominated by the contribution fiom CO. However, noticeable differences are seen in the relative intendties of bands S and T: the contribution fiom mainly C 2s of the substituted ligands should be considered for phosphine and norbodene complexes. For the first time in these phosphine complexes, the spin orbit components of phosphorus 2p have been resolved and their second
order ionizations been observed. Spin-orbit splittings, ligand field efFects and vibrational stmctures are observed in the spectra of both W 5d and W 4f regions. As the CO ligands are systematicaliy replaced by phosphines on the metal center, al1
the metal and l i g d orbitals SM?. The changes in charge potential cause t hese shifts towards lower energy because phosphine is a stronger o donor and weaker x acceptor than CO.
Meanwhüe, the difference in x acceptor ability of the ligands lads to ligand field splittings of the metal t,orbitals (since phosphine is a weaker x acceptor than CO). Ligand field splittings on both the W 5d and W 4f levels increase in the order of W(CO), = futW(CO),(PMt&
