Transition Metal Coordination Chemistry

Transition Metal Coordination Chemistry

What is a transition metal?

Prof S.M.Draper 2.05 SNIAMS Building [email protected]

Recommended books

M.J. Winter, d-block Chemistry, Oxford Chemistry Primers, OUP, 2001 M.S. Silberberg, Chemistry, 3rd Ed, McGrawHill, 2003 (chapter 23)

C.E. Housecroft, A.G. Sharpe, Inorganic Chemistry, 1st Ed, PrenticeHall, 2001 J.E. Huheey, E.A. Keiter, R.L. Keiter, Inorganic Chemistry, 4th Ed., HarperCollins, 1993

1

Electrons in atoms

z Shapes of d-orbitals

4s

y x

3d

4p

Sc Ti

z z

z

V

y Cr

y

y

Mn

x

x

x

Fe Co

yz

xz

xy

x2-y2

z2

Ni Cu Zn

2

Working out numbers of d-electrons from oxidation states:

How many d-electrons has the metal?

1 2 3 4 5 6

7 8

9 10 11 12

1st: how many electrons are there in the shell? O

- count along the periodic table e.g. Mn = 7 electrons

ox =

Cu = 11 electrons

en =

O

-O

H2N

O-

NH2

2nd: how many electrons are lost? - oxidation state

complex

O.S. of L

O.S. of M

no. d electrons

e.g. Mn(VII) = 7 electrons lost Cu(II) = 2 electrons lost

[Cr2O7]2-

-2

+6

d0

[MnO4]-

-2

+7

d0

0

+3

d1

-1, 0

+2

d8

-2

+3

d5

[Ag(NH3)2]+

3rd: how many electrons left over? - subtract e.g. Mn(VII) =

[Ti(H2O)6]3+ Cu(II) =

[Co(en)3]3+ [PtCl2(NH3)2] [V(CN)6]4-

Hence the only valance electrons available in a transition metal ion are d-electrons

[Fe(ox)3]3-

3

Colour of transition metal complexes biological activity

magnetic behaviour colour

Ruby Corundum Al2O3 with

impurities

geometry

What’s interesting about

Sapphire

Transition Metal Complexes??

Al2O3 with

coordination number

medical applications oxidation states

octahedral metal centre

Corundum and

impurities

coordination number 6

Emerald Beryl AlSiO3 containing Be with

impurities

4

Haemoglobin

Oxygen carrier in blood O2

Porphyrin-Fe transition metal complex

Cisplatin [PtCl2(NH3)2]

Fe(II) ion is octahedrally coordinated NN NN Fe NN Fe NN 2C HOHO 2C

Coordination number 6

NR NR HO C

2 HO 2 C

square planar Pt(II) coordination number 4 cis-isomer

the first of a series of platinum coordination complex-based anti-cancer drugs (Platinol-AQ)

5

Alfred Werner - Nobel Prizewinner 1913

[Co(NH3)6]Cl3

[Co(NH3)5Cl]Cl2 3+

CoCl3 . 6NH3

yellow

xs Ag+

3 moles AgCl

CoCl3 . 5NH3

purple

xs Ag+

2 moles AgCl

CoCl3 . 4NH3

green

xs Ag+

1 mole AgCl

xs Ag+

0 moles AgCl

CoCl3 . 3NH3

[Co(NH3)4Cl2]Cl +

2+

Werner's conclusions

1.

The metal is in a particular

(primary valancy)

2.

The complex has a fixed

(secondary valancy)

3.

The ligands are bound to the metal via a bond which resembles a covalent bond

6

What is a coordination complex?

Lewis Acid-Base Concept: parallel with main group chemistry • Adducts formed by the reaction of metal ions (Lewis acids) with several ligands (Lewis bases). H Ni2+

+

H 2O

H

+

6 :NH 3

X+/-

2+ O . .:

H 3 N: NH 3

n

Ni NH 3

Ni 2+

n+/-

Ni

NH 3

2+

:NH 3

Central metal ion or atom surrounded by a set of ligands NH 3

The ligand donates two electrons to the d-orbitals around the metal forming a

• Properties of products differ from those of separate reactants (i.e., colour, composition, magnetic properties, etc.).

1 3

7

F

F zz

F

B

z z

F

F

B

z z

F

z z

H

F

B

F

H

N

H

zz

H

F

N H

H NH3

_ H N > BF

BF3

3

zz

6

L

H

N H

L

L

3+

zz

3

zz

NH3

e.g. [Co(NH3)6]3+

z z

+

zz

z z

zz

H

H

z z

z z

zz

N

z z

H

z z

zz

z z

H

+

Co3+

H3N H3N

z z

z z

z z

NH3 z z

NH3

zz

L

L

NH3

L = coordinate or dative bond

1

C

Group 14

O

CO carbon monoxide e.g. NH3, H2O, OH-, CO32-

CO, PPh3, C2H4, SRH, CN-, SCN-

Small donor atoms Electronegative

H H Group 15

N

NH3 ammonia

O

H2O water

H

PPh3 phosphine

S

SR2 thioether

Not very polarisable

Less electronegative Easily polarisable

R

H Group 16

P

Larger donor atoms

H

R

e.g. Fe(III), Mn(II), Cr(III)

e.g. Ag(I), Cu(I)

Small metals (1st row)

Larger metals (2nd + 3rd row)

High oxidation state

Low oxidation state

2

π - bonded ligands

Group 14

C

N

z z

CNcyanide

-

Phphenyl

CH2

Cl K+

Cl

H2C

CH2

RC

CR

Pt Cl CH2

Group 15

N

O

z z

Group 16 O

Group 17

H

X

NOnitrous

N

OHhydroxide

S

halide

C

C

H

S

NCS-

z z

N

isocyanate

z z

[PtCl3(η2-C2H4)]-

SCNthiocyanate

hydride

Monodentate

donor atom per ligand

Bidentate

donor atoms per ligand

Tridentate

three


Multidentate

many


3

Anionic bidentate ligands

Neutral bidentate ligands: 2 donor atoms

O H 2N

NH2 zz

zz

e.g. [PtCl2(en)]

π-donor bidentate ligand

O

O

O

oxalate = ox2-

-

Fe C

C

O

O

O O

1,2-diaminoethane = ethylene diamine = en

[Fe(CO)3(η4-C4H6)]

acetate = ac-

H 3C

C

O

Ph2P

zz

PPh 2 zz

N zz

N zz

N

zz

zz

2,2'-bipyridine bpy

1,10-phenanthroline phen

R'

O

N

O

O

Pd R'

1,2-diphenylphosphineethane dppe

OH

R

N

M

O

N HO

R

C N H

N C H

Pd(II)-oxime complex

4

Tetradentate ligands

Tridentate ligands: three donor atoms

Hexadentate ligand tetraanion of

H 2N

zz

zz

NH2

N

NH2

NH

zz

N

zz

zz

ethylenediaminetetraacetic acid

N N

NH2

zz

EDTA NH2

O

tris(2-aminoethyl)amine diethylenetriamine

2,2':6',2"-terpyridine

dien

tpy

O -

tren

O

N

1,2,4-triazacyclonane

HN

NH

zz

NH N

HN

N

O

N

O O

-

O

N N

N

zz

macrocyclic ligand

NH

N

O

N

HN N

zz

N H

M

porphyrin

phthalocyanin

5

Inner coordination sphere = Outer coordination sphere =

H2O

SO42-

H2O H2O

Mn2+

OH2 OH2

H2O

[Mn(OH2)6][SO4]: outer sphere complex

H2O OSO3

H2O Mn2+

OH2

H2O H2O

[Mn(OH2)5(SO4)5]: inner sphere complex

6

Ligand Exchange Reactions [Fe(OH2)6]2+ 2+

[Fe(OH2)5(CN)]+

+

H2O

+ +

hexaaquo iron(II) complex

CN-

+

cyanide ion

+

K1

=

const [H2O]

=

[Fe(OH2)5(CN)+] [H2O] [Fe(OH2)62+] [CN-]

1

The reaction continues….

The reaction continues…. [Fe(OH2)5(CN)]+

+

K2

=

CN-

[Fe(OH2)4(CN)2]+

+

H2O

[Fe(OH2)4(CN)2] + CN-

[Fe(OH2)3(CN)3]-

+

H2O

β3 =

[Fe(OH2)3(CN)3]- + CN-

[Fe(OH2)2(CN)4]2- +

H2O

β4 =

[Fe(OH2)2(CN)4]2- + CN-

[Fe(OH2)(CN)5]3-

+

H2O

β5 =

[Fe(CN)6]4-

+

H2O

β6 =

[Fe(OH2)4(CN)2] [Fe(OH2)5(CN)+] [CN-]

K1K2

=

β2

[Fe(OH2)5(CN)+] = [Fe(OH2)62+] [CN-]

x

[Fe(OH2)3(CN)3-] [Fe(OH2)62+] [CN-]3 [Fe(OH2)2(CN)42-] [Fe(OH2)62+] [CN-]4

[Fe(OH2)(CN)53-] [Fe(OH2)62+] [CN-]5

[Fe(OH2)4(CN)2] [Fe(OH2)5(CN)+] [CN-]

[Fe(OH2)(CN)5]

+ CN-

Overall Stability Constant

βn

=

[Fe(CN)64-] [Fe(OH2)62+] [CN-]6

[MLn] [M] [L]n

Stability constants are often expressed in log form ie log βn

2

Values of βn

The Chelate Effect

2+

4-

+ 6

+ 6

[M(NH3)6]2+

n+

NH3 H3N

M

NH3 NH3

H3N NH3

β6

=

[Fe(CN)64-] [Fe(OH2)62+] [CN-]6

log β6 = 35

n+

H2N H2 N N H2

[M(en)3]2+

NH2

M

NH2 H2N

~ 1035 [M(OH2)6]n+

+

6 NH3

[M(NH3)6]n+ +

6 H2O

[M(OH2)6]n+

+

3 en

[M(en)3]n+

6 H2O

+

Kn < … < K3 < K2 < K1

3

ΔGo = ΔHo - TΔSo

Entropy of chelate formation ΔGo = -RT ln K [Cu(OH2)6]2+ +

β2107.7

ΔGo = ΔHo - TΔSo

Enthalphy changes similar

ΔHo

= - 46 kJ

+

[M(OH2)6]n+

+

mol-1

6 NH3

[M(NH3)6]n+

+

6 H2O

3 en

[M(en)3]n+

+

6 H2O

2 H2O

+

2 H2O

log β2 = 7.7 ΔSo = - 8.4 J K-1mol-1

[Cu(OH2)4(en)]2+

en

β2 1010.6 log β1 = 10.6 ΔHo = - 54 kJ mol-1

ΔSo is large and positive

+

Entropy changes differ [Cu(OH2)6]2+ +

[M(OH2)6]n+

[Cu(OH2)4(NH3)2]2+

2 NH3

ΔSo = + 23 J K-1mol-1

- TΔSo is large and negative

4

Thermodynamic vs Kinetic Stability The Macrocyclic Effect Complexes containing macrocyclic rings have enhanced stability compared to acyclic ligands

2+ H

N

N

H

2+ H

N

H

Cu

Cu N H2

N

N H2

log K1 = 23.9 acyclic chelating ligand

H

N

N

H

log K1 = 28.0 macrocyclic ligand

e.g

[Cr(OH2)6]3+

d3

kinetically inert

slow substitution of Ls

cf.

[Fe(OH2)6]3+

hs d5

kinetically labile

rapid substitution of Ls

(thermodynamic stability similar)

ΔG always favours formation of macrocyclic complex

5

Tour of Coordination Numbers and their Geometries

Most Common Geometries of Transition Metal Complexes

The Kepert Model The shape of a complex is usually dictated by the number of coordinated atoms. The coordinated atoms are as far away from each other as possible.

Tetrahedral

109o 28'

C.N. 4

Square Planar

90o

C.N. 4

Trigonal bipyramidal

120o + 90o

C.N. 5

Square based pyramidal

90o

C.N. 5

Octahedral

90o

C.N. 6

Non-

bonding electrons are ignored.

Note: •

regular geometries are often distorted

•

structural features of multinuclear complexes are described in terms used for individual metal centres

•

when energy differences between different structures are small, fluxional behaviour may be observed

6



Cu(I), Ag(I), Au(I), Hg(II)

[CuCl2]-

trigonal bipyramid

[Au(CN)2]-

square-based pyramid

180o 90o


90o

120o

[Cu(CN)2]-

[HgI3]CN

CN

Cu

Cu C

N

N

C

C

120o

N

N

C

Cu

Cu

CN

CN

The two structures have very similar energy

n

7

trigonal bipyramid 3-

Cl Cl

Cu

square-based pyramid

Cl NC

Cl

CN CN

[Co(CN)5]3-

[CuCl5]3-

+

N Co

Co

NC

Cl

N

3-

CN

O N N

Br

[Co(Me6tren)Br]+

O O

V

O O

[VO(acac)2]

8

Tetrahedral complexes

Favoured by steric requirements

optical isomerism 1

1 2

4 3

[CoCl4

4 3

]2-

[MnO4]90o

2

non-superimposable mirror images

[NiCl4]2-

109o

1

Square Planar Geometry

Square planar geometry

e.g. [PtCl4]2[AuBr4][Co(CN)4]2-

cisplatin

Square planar complexes are formed by i.e. group 10

Ni2+, Pd2+, Pt2+ Au3+

cis-[PtCl2(NH3)2]

trans-[PtCl2(NH3)2]

cis-diamminedichloroplatinum(II)

trans-diamminedichloroplatinum(II)

Square planar complexes are formed by e.g. CN-

2


Octahedral geometry

Geometrical Isomerism

[ML4X2]

e.g. [Mn(OH2)6]2+ [Cr(CO)6]

Staggared ligands vs. Eclipsed ligands

C+

trans-[Co(NH3)4Cl2]+

cis-[Co(NH3)4Cl2]+

green

violet

C+

do metals e.g. WMe6

3

Octahedral geometry [ML3X3]

Geometrical Isomerism

Octahedral geometry [M(Κ2L)2X2]

Geometrical and Optical Isomerism

e.g. [Co(en)2Cl2]+

trans geometrical isomer H N

Cl

H N

Co N H

Cl

N H

mer-[Co(NH3)3(NO2)3] cis geometrical isomer

fac-[Co(NH3)3(NO2)3]

two optical isomers

Non-superimposable mirror images = Δ and Λ enantiomers

4

Octahedral geometry

Optical Isomerism

Tetragonal Distortion: Tetragonal elongation z-axis

2+ [M(Κ2L)3]

2 long axial & 4 shorter equatorial bonds

N

N

Ru

N

N N

N

[Cu(OH2)6]2+, [Cr(OH2)6]2+

Tetragonal compression z-axis

M [Ru(bpy)3]2+

2 short axial & 4 longer equatorial bonds trans-[Co(NH3)4(CN)2]+

* Small distortion but symmetry lower than octahedral.

5

Isomerism

Trigonal Distortion: • Two faces on opposite sides of octahedron move either towards or away from each other

N N Rotate by 60°

Elongation

N

Stereoisomerism

Isomers have the same empirical

Isomers have the same M-L bonds,

formula but the atoms connectivities

but the atoms are arranged differently

differ

in space

Ionisation isomerism

Geometrical isomerism

M N N

Complexes of tacn can show this type of distortion [M(tacn)2]2+

N Hydration isomerism

cis/trans

Coordination isomerism

mer/fac

Linkage isomerism

N N

Structural Isomerism

N

tacn = 1,3,7-triazacyclononane

Polymerisation isomerism

Optical isomerism Δ and Λ

6

Ionisation isomerism

Coordination isomerism

Exchange of a ligated anion with a counterion

no ppt

[Co(NH3)5Br]SO4 Ag+ [Co(NH3)5(SO4)]Br

AgBr

Ba2+ Ba2+

[Cu(NH3)4][PtCl4]

[Pt(NH3)4][CuCl4]

square planar

[Co(NH3)6][Cr(CN)6]

[Cr(NH3)6][Co(CN)6]

octahedral

BaSO4 no ppt Polymerisation isomerism

Solvate isomerism Exchange of a neutral ligand for an anionic ligand e.g.

[Co(OH2)6]Cl3

e.g.

[Pt(NH3)4][PtCl4]

[Pt(NH3)2Cl2]

violet

[Co(OH2)5Cl]Cl2

light green

[Co(OH2)4Cl2]Cl

Both polymers have the empirical formula [Pt(NH3)2Cl2]n

dark green

7

Higher Coordination Numbers

Linkage isomerism

Coordination Number 7 hν [Co(NH3)5(NO2)]2+

Δ

[Co(NH3)5(NO2)]2+

yellow

red

nitro-complex

nitrito-complex

[Pd(NCS)2(PPh3)2] isocyanate

[Pd(SCN)2(PPh3)2] thiocyanate

[WBr3(CO)4)](distorted)

[TaF7]2[ZrF7]3-

8

Coordination Number 8

Na3[Mo(CN)8]

(nBu4N)3[Mo(CN)8]

Coordination Number 9

[ReH9]2-

9

Symmetrical ligand field

z

Shapes of d-orbitals

orthogonal z

y x

M

x and y

z

z

z

y y

y x

x

x

x2-y2 yz

z2

xz xy

Energy yz

xz

xy

x2-y2

z2 x2-y2 yz

z2

xz xy

1

Effect of ligand field on metal d-orbitals

The difference in energy between the eg and the t2g energy levels is the . This is given the value 10 Dq.

symmetrical field

octahedral ligand field

eg Δo or 10 Dq

t2g

2

Electron configurations: d1 ion

3+

The nature of Δoct [Ti(OH2)6]3+

e.g. [Ti(OH2)6]3+

490-580 nm

violet solution in water

one d-electron in a t2g orbital the complex has a Crystal Field Stabilisation Energy (CFSE) of - 0.4 Δoct Absorption spectrum: λmax = 510 nm = 243 kJ mol-1

3

d4 ions e.g. [V(OH2)6]3+

e.g. [Cr(OH2)6]3+

+ 0.6 Δoct

CFSE =

- 0.4 Δoct

= - 0.6 Δoct

+ 0.6 Δoct CFSE = - 0.4 Δoct

= - 1.6 Δoct + P

4

Factors affecting the magnitude of Δoct

High and Low spin Complexes High spin complex

Low spin complex

e.g.

Δ is small

Δ is large

electrons occupy eg and t2g orbitals

electrons pair in t2g oribtals before

singly before pairing

occupying eg orbitals

[Fe(OH2)6]2+

Δoct = 10 000 cm-1

[Fe(OH2)6]3+

Δoct = 14 000 cm-1

[Co(OH2)6]2+

Δoct = 9 700 cm-1

[Co(OH2)6]3+

Δoct = 18 000 cm-1

e.g.

[Co(NH3)6]3+

Δoct = 22 900 cm-1

[Rh(NH3)6]3+

Δoct = 34 100 cm-1

[Ir(NH3)6]3+

Δoct = 41 000 cm-1

5

eg eg I- < Br- < S2- < SCN- ≈ Cl-< NO3- < F- < OH- < ox2t2g

< H2O < NCS- < CH3CN < NH3 ≈ en < bpy < phen ≈ NO2- < PR3 < CN- ≈ CO

t2g

M.J. Winter, d-block Chemistry, p.83

6

Octahedral Crystal Field

ions, Oh field

d-orbital energies in…

free ion

spherical ligand field

+ 0.6 ∆oct

octahedral ligand field

- 0.4 ∆oct

x2-y2 yz

z2 xz

xy + 0.6 ∆oct

x2-y2 yz

z2 xz

- 0.4 ∆oct

xy

the complex has a Crystal Field Stabilisation Energy (CFSE)

ions, Oh field

ions, Oh field High Spin eg

eg

+ 0.6 ∆oct

+ 0.6 ∆oct t2g

t2g

- 0.4 ∆oct

- 0.4 ∆oct

Low Spin eg + 0.6 ∆oct t2g

- 0.4 ∆oct

+ 0.6 ∆oct - 0.4 ∆oct

1

ions, Oh field

ions, Oh field

eg

eg 1. What is the CFSE of [Fe(CN)6]3-?

eg + 0.6 ∆oct

+ 0.6 ∆oct

- 0.4 ∆oct

t2g

- 0.4 ∆oct

t2g

eg. 2. If the CFSE of [Co(H2O)6]2+ is -0.8 ∆oct, what spin state is it in?

ions, Oh field

t2g

+ 0.6 ∆oct

Only

- 0.4 ∆oct

can be high or low spin

Jahn-Teller Distortion

configurations


z

z-out: d-orbitals with z-component are stabilised y Oh field

x

+ 1/2 δ1

eg

- 1/2 δ1 ∆oct + 2/3 δ2 t2g The complex

ligands contract or elongate giving a

- 1/3 δ2

2


Jahn-Teller Theorum

z-in: d-orbitals with z-component are destabilised

"For a nonlinear molecule in an electronically degenerate state distortion must occur to lower the symmetry, remove the degeneracy and lower the energy"

Oh field

x2-y2 + eg

x2-y2 z2

1/ 2

δ1

eg

x2-y2 z2 z2

- 1/2 δ1 t2g

yz

xz

xy

+ 1/3 δ2 yz

xz

Oh

z-in

Na2[CuF4] Oh

z-out

Cr2F5

z-out

xy

∆oct

t2g

e.g. K2[CuF4]

Oh

Cr(II) = high spin d4 yz xz

xy - 2/3 δ2

e.g. [Cu(OH2)6]2+ Cu(II) = d9 z-out

Square planar geometry: ML4 Oh

z-out

square planar x2-y2

eg

x2-y2 x2-y2 z2

z2 xy

t2g

xy yz

xz

z2

xy yz xz yz xz d8 complexes: some Ni(II), and Pd(II), Pt(II)

3

Tetrahedral crystal field splitting

Splitting of d-orbitals in a tetrahedral crystal field eg

Δoct metal ion in free space

in symmetrical field

t2g

x2-y2 yz z2 xz xy

1

Effect of crystal fields on complex geometry

Spin orbit coupling is particularly significant for metals lower in the periodic table but for 1st row transition metal complexes the formula can be used in a simplified form because: “The spin contribution outweighs the orbital angular momentum contribution to μeff" Spin only magnetic moment, μSO

μSO =

n (n + 2)

where n = number of unpaired electrons

μSO = 2

S (S + 1)

where S = total spin quantum number = n / 2

2

Spin only magnetic moment, μSO Measurement of magnetic susceptibility μSO =

n (n + 2)

where n = number of unpaired electrons Gouy Balance

What is the spin only magnetic moment of …..? eg x2-y2

z2

[Co(OH2)6]2+ n=

pivotal balance

μSO =

=

yz

xz

t2g xy

yz

xz

t2 xy

[NiCl4]2μSO =

z2

= x2-y2

[Ni(CN)4]2n=

sample

e x2-y2

n=

scale

N

S

magnetic field

xy

μSO =

z2

= yz

xz

3

Values of μeff for high spin Oh complexes Effective magnetic moment, μeff From experimentally measured molar magnetic susceptibility, χm

T is temperature in Kelvins

=

eh = 4 π me

9.27 x 10-24 J T-1

0

0

0

d1

1/ 2

1.73

1.7 - 1.8

d2

1

2.83

2.8 - 3.1

d3

3/ 2

3.87

3.7 - 3.9

Ti3+

Cr3+

Cr2+,

μB

d0

Sc3+, Ti4+

V2+,

2.83 is the magnetogyric ratio

μSO(BM)

dn

V3+

μeff in Bohr Magnetons:

S

M ion

μobs (BM)

Mn3+

d4

2

4.90

4.8 - 4.9

Mn2+, Fe3+

d5

5/ 2

5.92

5.7 - 6.0

Fe2+, Co3+

d6

2

4.90

5.0 - 5.6

Co2+

d7

3/ 2

3.87

4.3 - 5.2

Ni2+

d8

1

2.83

2.9 - 3.9

Cu2+

d9

1/ 2

1.73

1.9 - 2.1

Zn2+

d10

0

0

0

4

Colour in TM complexes

Colour of d-d transitions depends on magnitude of Δ

[Ti(OH2)6]3+

Absorption spectrum: λmax = 510 nm

white light 400-800 nm

490-580 nm

The Colour Wheel If red light is absorbed

Blue: 400-490 nm yellow-green: 490-580 nm Red: 580-800 nm

the complex appears green If purple light is absorbed the complex appears yellow wavelength, λ (nm)

5

Effect of magnitude of Δ on colour

Effect of magnitude of Δ on colour

1. For a given ligand, the colour 2. For a given metal ion, the colour depends on [V(H2O)6]3+

[V(H2O)6]2+

violet light absorbed

yellow light absorbed

complex appears yellow

complex appears violet

3+

2+

6

Colour and the Spectrochemical Series

Weak Field Ligands

Strong Field Ligands

High Spin Complexes

Low Spin Complexes

I- < Br- < S2- < SCN- < Cl-< NO3- < F- < OH- < ox2small Δ

< H2O < NCS- < CH3CN < NH3 < en < bpy

large Δ

< phen < NO2- < phosph < CN- < CO

7

Transition Metal Coordination Chemistry

[Ti(H2O)6]3+

Absorption spectrum: λmax = 510 nm

490-580 nm

Professor Sylvia Draper eg

[email protected] WHERE HAVE WE BEEN ?

Lecture 7: Magnetism and Colour Tetrahedral crystal field Magnetism Spin only magnetic moment Magnetism measurements Absorption of light d1 complexes and colour Effect of ∆ on colour

WHERE ARE WE GOING ?

hν t2g

eg ∆o

t2g

Lecture 8: Colour continued, and Ligand Field Theory Selection rules Limitations of CFT LFSE and ∆

Factors affecting magnitude of ∆ 1.

Oxidation state of metal e.g.

2.

Position of metal in periodic table

3.

Type of ligand in spectrochemical series

Thermodynamic aspects of LFT

1

Intensity of colour depends on …..

The Intensity of the colour depends on Selection Rules and is measured by the absorbance Absorbance is defined by the Beer-Lambert equation: A = log10(I0/I) = ε.c.l where A = absorbance I0 = intensity of incident light; I = intensity of light after passing through the cell; ε = molar absorption coefficient; c = concentration; l = cell pathlength. Typical Spectrum: Octahedral Cr(III) complex

Laporte Selection Rule

2.

Spin Selection Rule

At 590 nm, A = 0.138 A = εcl 0.138 = ε x 0.002M x 1 cm ε = 0.138/ 0.002 x 1 ε = 69 M-1 cm-1

Absorbance

1.

Calculation of ε:

0.25

?

[Cr] = 0.002M

0.2

0.15 0.1 0.05 0

300 350 400 450 500 550 600 650 700 Wavelength (nm)

Transitions allowed by the selection rules (high ε values > 1000

M-1cm-1).

2

Laporte Selection Rule

Transitions between d-orbitals (g) forbidden by the Laporte selection rule

Octahedral complexes Centrosymmetric: t2g and eg orbitals

Selection rule lifted by molecular vibration

s-orbital gerade

e.g.

d-orbital gerade

p-orbital ungerade

p-orbital to d-orbital

by the Laporte selection rule

d-orbital to d-orbital

by the Laporte selection rule

Tetrahedral complexes non-Centrosymmetric: t2 and e orbitals

Tetrahedral complexes are usually more strongly coloured than analogous octahedral complexes

3

The Spin Selection Rule

Assumptions made in CFT 1.

The ligands are

2.

Interactions are purely

1.

Geometry

2.

Magnetism

3.

Colour

Explains

Limitations Does not allow for

in M-L bonding

Doesn't explain Order of ligands in the Spectrochemical Series Nephelauxetic Effect

4

The Spectrochemical Series

The Nephelauxetic Effect

Weak Field Ligands

Strong Field Ligands

High Spin Complexes

Low Spin Complexes

"there is

in complexes than in the free ion"

- some I- < Br- < S2- < SCN- < Cl-< NO3- < F- < OH- < ox2- < H2O < NCS- < CH3CN < NH3 < en < bpy < phen < NO2- < phosph < CN- < CO

in M-L bonds – M and L share electrons

- effective size of metal orbitals - electron-electron repulsion

Nephelauxetic series of ligands F- < H2O < NH3 < en < [oxalate]2- < [NCS]- < Cl- < Br- < ICFT does not explain why some ligands with

charges are

than analogous ligands which are not charged

Nephelauxetic series of metal ions Mn(II) < Ni(II) Co(II) < Mo(II) > Re (IV) < Fe(III) < Ir(III) < Co(III) < Mn(IV)

5

LFSE:the EXTRA stabilisation of complexes that occurs as a result of the splitting of orbitals in a ligand field (cf. to CFSE: it is always positive and no account of P is taken) LFSE Octahedral (always greater than tetrahedral) high/low spin options d4 to d7 only

eg

d1

d2

d3

d1 : LFSE 0.4 3/5∆o -2/5∆O

t2g

d2 : LFSE 0.4 X 2 = 0.8 d3 : LFSE 0.4 x 3 = 1.2 maximum d4 hs : LFSE 0.4 x 3 – 0.6 = 0.6

d4

d4

d5

eg

LFT is identical to CFT, except that it allows for some

in M-L bonding.

3/5∆o -2/5∆O

t2g

parameters.

d5 hs : LFSE 0.4 x 3 – 0.6 x 2 = 0 minimum d5 ls : LFSE 0.4 x 5 = 2.0 highest d6 hs : LFSE 0.4 x 4 - 0.6 x 2 = 0.4

d6

LFT is based on

d4 ls : LFSE 0.4 x 4 = 1.6

d5

eg

t2g

d6

d7

d6 ls : LFSE 0.4 x 6 = 2.4

d7 3/5∆o -2/5∆O

d7 hs : LFSE 0.4 x 5 – 0.6 x 2 = 0.8 d7 ls : LFSE 0.4 x 6 – 0.6 = 1.8 d8 : LFSE 0.4 x 6 – 0.6 x 2 = 1.2 maximum

ls gives higher LFSE than hs option

6

Variation in LFSE with ∆excluding P)

Lattice Energies (from Born Haber cycle) For 1st row M(II) chlorides: hs, Oh

Ca(II)

ls d6:

LFSE

hs d0, d5, d10 :

LFSE

hs Oh d3, d8 :

LFSE

Ti(II)

V(II)

The energy

Cr(II)….etc

when ions come together from infinite

separation to form a crystal Mn+(g) + n X-(g)

MXn(s)

Discrepancies in lattice energies are a result of

7

Variation in Hydration Enthalpies

Enthalpy of hydration: M2+(g) + xs H2O

[M(OH2)6]2+(aq)

8

Variation in Ionic Radii

Ca2+

Zn2+

Sc3+

Ga3+

high spin low spin

Last Lecture 9 lecture course on Coordination Chemistry Prof. Sylvia Draper high spin low spin

So……. CFSE, LFE and MO theory agree ! - each one building upon and improving the other

low spin: steady

followed by increase from t2g6 eg1

high spin: steady

followed by increase from t2g3 eg1

1

Ionisation Energies of first row TM ions, hs, Oh

Irving-Williams series

[M(H2O)6]2+

+

[EDTA]4-

[M(EDTA)]2- +

6H2O

Position of equilibrium depends on

d5

d6

d7

d8

d9

d10

Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+

2

Valance Bond Theory

Tetrahedral e.g. [Zn(OH)4]2-

Square Planar e.g. [Ni(CN)4]2-

Ligand = Lewis base Metal = Lewis acid s, p and d orbitals give

with specific geometries

Number and type of M-L hybrid orbitals determines geometry of the complex

Octahedral Complex e.g. [Cr(NH3)6]3+ Limitations of VB theory Assumes bonding is Does not account for Can predict

wrongly

Does not account for

3

Molecular Orbital (MO) theory

Ligand Group Orbitals

MO theory considers covalent interactions e.g. molecular hydrogen

Metal valance shell orbitals =

Linear Combination of Atomic Orbitals (LCAO)

Ligand valance shell orbitals =

LCAO approach

1s

e.g. [Co(NH3)6]3+ = sigma-bonded complex 1s

1s

zz

2s

2p

sp3 hybrid orbitals

N H

H atom

H2 molecule

H atom

H H

Six sp3 hybrid ligand orbitals form a set of

4

e.g. [CoF6]3-

Co3+ = 6 e-

t1u*

6 x F- = 12 ea1g*

Complexes with M-L π-bonding e.g. H2O,OH-, lower halides Cl-, Br-, I-

Total = high spin

electron density to the metal centre ligand orbital and

4p

π-bond σ-bond π-bond

metal orbital of electron density from filled p orbitals

4s eg* t2g

3d

6 x LGO eg

π-acceptor ligands e.g. CO, N2, NO, alkenes electron density from the metal centre metal orbital and

t1u

O

C

C

O

ligand orbitals

L

Mn+

a1g ML6n+

L L

L L L

of electron density into emtpy π∗-antibonding orbitals

5

π-acceptor ligands: back-donation to empty ligand antibonding orbitals t1u*

π-donor ligands: filled ligand orbitals and empty metal orbitals t1u* a1g*

small ∆oct - weak field ligand

a1g*

Ligand π∗-orbitals

eg*

4p

large ∆oct - strong field ligand

4p Ligand π-orbitals

4s

(vacant)

eg*

4s

(occupied) 3d

3d Ligand σ-orbitals

Ligand σ-orbitals

(occupied)

(occupied)

eg Metal d electrons (d6 example)

Metal d electrons

t1u

(d6 example)

a1g Energy

Mn+

ML6n+

eg t1u a1g

6L

Energy

Mn+

ML6n+

6L

6