Optical transitions in D4h Copper chloride complexes

OPTICAL TRANSITIONS IN D4h COPPER CHLORIDE COMPLEXES : AN MS-Xα INVESTIGATION by J.A. ARAMBURU and M. MORENO D.C.I.T.T.Y.M., Sección Ciencia de Materiales, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain (Reçu le 29.9.88, accepté le 22.11.88)

ABSTRACT The crystal-field (CF) and charge transfer (CT) spectrum of the square-planar CuCl42-" complex as well as the ones relevant to the elongated and compressed CuCl64- complexes have been investigated through the MS-Xa methodology. The calculations performed for different values of the equatorial and axial distances Req and Rax, reproduce well the experimental values found for CuCl42- (Req=226.5 pm) and CdCl2:Cu2+· (Req=235 pm; Rax=280 pm). Although an increase of Req decreases the energy of the first CT transition, CdCl2:Cu2+ exhibits a value close to that of CuCl42- due to the presence of axial ligands which rises the energy of the d-orbitals. The large Req distance in CdCl2:Cu2+ is reflected however in the smaller separation between the two eu(π+σ)—>x2-y2 and eu( σ+ π )—>x2-y2 CT transitions. The present calculations indicate that the optical spectrum detected for (en H)2 MnCl4:Cu2+ displaying the first CT transition at 20800 cm-1 is hard to be ascribed to an elongated CuCl64- complex. By contrast it can reasonably be understood assuming a compressed situation.

RESUME Les transitions de transfert de charge (CT) et de champ cristallin (CC) pour le complexe carré plan CuCl42- et des complexes octaédriques CuCl64- distordus sont évaluées en utilisant les états de transitions dans la méthode MS-Xα; la dépendance des transitions en fonction des distances metal-iigand est étudiée de manière à obtenir les structures électroniques des sels des cristaux où ces complexes interviennet. En particulier, le spectre optique de (enH)2MnCl4:Cu2+ est attribué à un complexe CuCl64- distordu ou les deux Cl axiaux sont plus proches de Cu2+ que les quatre Cl- équatoriaux.

Journal de chimie physique, 1989, 86, n°4

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INTRODUCTION Aside from the simplicity of their electronic structures Cu2+ complexes with D4h symmetry are attractive because they give rise very often to two kinds of optical bands (the crystal-field (CF) and charge-transfer (CT) bands) and at the same time can also be explored through the EPR and ENDOR techniques. The present work is devoted to a theoretical investigation of several chloride complexes with

symmetry by means of the MS-Xα methodology. A

particular attention will be paid through this work to the dependence of CT and CF transitions on metal-ligand distances. Aside from the pure square-planar CuCl42- found in some salts (like (creat)2 CUCl4 or (met H)2 CUCl4) and which has received very recently a significant attention [1-10] we shall also study the "elongated" CuCl64- complex observed in some octahedral lattices (like CdCl2, LiCl or NaCl) doped with Cu2+ [11 -15] . Also CuClg4- complexes appear in (C2H5NH3)2 CUCl4 [2] and CSQlCl3 [16] salts. In the case of CdCl2:Cu+2 CT [11] as well as CF [12] bands have been detected. A main goal of the present work is just to understand the similarities and differences between the optical spectra of CuC4 2 - and CuCl44- on the basis of the two additional ligands present in the latter system as well as of a higher "equatorial" Cu2+-Cldistance for CdCl2:Cu2+ as it was previously pointed out [4].

Finally we shall offer MS-Xα results on the compressed CuCl64complex. This is done in order to investigate wheather the optical spectrum found by Schmid et al. [17] in (en H)2 MnCl4:Cu2+ (rather different to those corresponding to CUCl42- or elongated CuCl64- units, specially in the CT region) can be ascribed to this very rare species or not.

Journal de chimie physique, 1989, 86, n° 4

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THEORETICAL

The standard version of the SCF-MS-Xa method is used [18] . The calculations were performed on a Data General MV-10000 computer, using the Cook5/Tamul program written by M. Cook, B. Bumsten and G. Stanley. In the initial atomic calculations performed to construct the starting molecular potential neutral atoms have been used. The SCF procedure was stopped when the relative change in the potential was 10-3 or smaller. The ratios of the sphere radii were fixed to the values obtained using the Norman procedure [19] . We allow the atomic spheres to overlap, the absolute values of the sphere radii being determined by imposing the condition that the virial ratio -2/ be exactly one. This method has been previously used

[10] for computing the CT transitions of CUCl42- corresponding to Req=225 pm and Req = 226.5 pm. The transition state procedure has been used for computing all optical transitions.

RESULTS AND DISCUSSION In Table I are collected the values of the CF and CT transitions obtained through the MS-Xα method for the square-planar CuCl42- complex at different "equatorial" Cu2+-Cl- distances, Req· For comparison purposes the experimental values corresponding to (N-mpH)2 CuCl4 [2,6] which has an average Req value equal to 226.5 pm. are also included. It can be seen that the present calculations reproduce well the experimental CT transitions while they give rise to CF energies slightly higher than the experimental ones as already


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TABLE I. MS-Xα results for the optical transition energies (in cm-1) of CuCl42- obtained at different Req values (in pm). The experimental data of (N-mpH)2 CuCl42- [2,6] correspond to Req=226.5 pm.

Experimental

Req

Transition

225

226.5

235

(N-mpH)CuCl42-

2 xy—>x22 -y2

15790

15420

13600

12480

2 xz;yz—>x22 -y2

15470

15070

13550

14300

3z2-r2- x 2-y2

20180

19840

17940

16990

2 2 a2g — >x2-y2

25200

24410

21120

23700

eu(π+σ)—>x2-y2

28870

27970

23270

26050

eu(σ+π)—>x2-y2

40250

39040

32580

37700

noted by Bencini and Gatteschi [3] . In this line the present calculations indicate 2 9 that the x y ->x2-y2 and xz, yz—>x29 -y9 2transitions are very close, although the

ordering is the opposite to that found experimentally. The present data indicate that when Req changes 1 pm from Req=226.5 pm CT energies experience more important changes (~600 cm-1) than CF ones (~200 cm- 1).

The behaviour predicted for CT transitions agres with

experience when data corresponding to (creat)2 CuCl4 (Req=225.0 pm)

Journal de chimie physique. 1989, 86. n°4

and

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(N-mpH)2CuCl4 are compared [10].

Table I points out that

dE/dReq is

negative for both CF and CT transitions. The absolute value of this slope however decreases significantly when Req increases for CT transitions.

Table II reports the results found for the elongated CuCl64- complex at different metal-ligand distances. Anyway we have only considered Req values higher than 226.5 pm because the presence of the two axial ligands in the complex induces Req values higher as that for CuCl42-.

In this sense

TABLE II. Results of MS-Xα calculations for the elongated CuCl64performed at different Req and Rax distances and experimental values for CdCl2:Cu2+ [11,12], eu(π+σ; eq) and eu(π;ax) mean bonding orbitals mainly built from equatorial and axial CT (3p) wave functions respectively. Distances are given in pm and transition energies in cm-1. Req=226.5

Req=235

Req=229

Experimental

Rax=280

Rax=280

Rax=288

CdCl2:Cu2+

9110

6740

9200

6370

xy—>x2-y2

13940

12090

13410

9400

xz;yz->x2-y2

13700

11770

13070

11000

eu(π+σ;eq)—>x2-y2 28160

26430

27440

25500

eu(π,ax)—>x2-y2

30570

28430

29500

eu(σ+π;eq)—>x2-y2 39580

35900

38040

Transition

3z2-r2- x 2-y2

33800


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Req=232 pm, Rax=278 pm for CuCl64- in CsCuCl3 [16] while Req= 228 pm, Rax=298 pm for (C2H5NH3)2 CuCl4 [2]. Moreover for CdCl2:Cu2+ it has been suggestted [4] Req=235 pm , Rax =280 pm from the analysis of the CF spectrum and isotropic superhyperfine tensor. The comparison between Tables I and II indicates that the presence of the two axial ligands strongly modifies the position of the 3z2-r2->x2-y2 CF transition. This reflects that the 3z2-r2 orbital is the one on which the repulsive effects due to the two "additional" ligands are the most intense. On the other hand the presence of the two axial ligands makes possible that the first allowed CT transition for the elongated CuCl64- complex lies at similar or slightly higher energy as for CuCl42altough Req is higher for the CuCl64- complex. In this way the first CT peak for CdCl2:Cu2+ [11] and LiCl:Cu2 + lie [14,15] at 25500 cm-1 and 28000 cm-1 respectively. This raising in the energy of the first CT band reflects an additional raising of the mainly 3d levels due to the two axial ligands. It is worth noting that the splitting, termedΔ (π,σ), between the eu( π+σ; eq)—>x2 -y2 and eu (σ+π; eq)—>x2-y2 transitions decreases significantly on passing from (N-mpH)2 CuCl4 (Δ(π,σ)=l 1650 cm-1) to CdCl2:Cu2+ (Δ(π,σ)=8300 cm-1) or LiCl:Cu2+ (Δ(π,σ)=8000 cm-1). This points out a higher Req value for the systems with elongated CuCl64- units and is well accounted for by the present MS-Xa calculations as indicated in Table II. In this line the comparison of the calculated values for Req=235 pm and Rax=280 pm with the experimental data for CdCl2):Cu2+ is satisfactory. It is worth noting that the eu (π; ax)—>x2-y2 transition involving a jump of one electron lying on axial ligands to Cu2+ is not experimentally detected. This is likely due to the fact that the axial l - (3p) wave


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functions do not appear in the mainly x2 -y2 antibonding level. Anyway further work on the oscillator strengths of these transitions is needed. An important consequence of the theoretical and experimental data reported in Tables I and II is the difficulty of finding the first CT band of a distorted CuCl64- complex at 20000 cm-l This situation could be obtained for a pure square-planar situation but only for Req values close to 240 pm which is about 15 pm higher than the equilibrium distance for CuCl42-. On the other hand the addition of the two extra axial ligands, though allows that Req may be close to 235 pm it also gives rise to an extra raising of the first CT band which makes that it should be placed above 25000 cm-1. Owing to this fact we have also explored wheather a compressed CuCl64complex can give rise to a first CT band at 20800 cm-1 as found in (en H)2 MnCl4:Cu2+ [17]. As in this case there are only two ligands in nearest neighbour position we have assumed that 210 pm < Rax < 230 pm while Req lies between 260 and 280 pm. The results given in Table III indicate that the appearence of a first CT band at 20800 cm-1 can reasonably be accounted for on the basis of a "compressed" CuCl64- complex. It should be pointed out that in this case the first CT band arises from a jump in which the electron goes from the equatorial ligands to the mainly 3z2-r2 orbital. In this way it is important to realize that for the compressed CuCl64- complex the 3z2-r2 orbital is able to produce σ-bonding with axial as well as with equatorial ligands. This situation is then quite different to that found for the elongated case. If a compressed situation is assumed for (en H)2 MnCl4:Cu2+ the large number of CT bands observed in the 20000-40000 cm-1 region as well as its

Journal de chimie physique, 1989, 86. n°4

- 878 TABLE III. MS-Xa results of the optical transition (in cm-1) for the compressed CuClg4- complex obtained for different values of axial (Rax) and equatorial (Req) Cu2+-Cl- distances (in pm).

Transition

Req=260 Req=270

Req=280

Req=280

Rax=230 Rax=23°

Rax=220

Rax=210

x2-y2_ >3z2_r2

4010

4360

5730

7070

xz;yz—>3Z2-r2

10480

9590

9120

9330

xy—>3z2-r2

11890

11110

11570

12920

eu(π+σ;eq)->3z2-r2

23740

21550

20800

21790

a2u(π;eq)—>3z22 -r22

25530

23480

23370

24800

eu(π+σ;eq+ax)—3 z2-r22 25930

23830

23380

24270

eu(π+σ;ax+eq)—>3z2-r2

34360

32220

33490

36280

a2u(π+σ;ax)—>3z2-r2

36170

34730

37570

41840

polarization [17] can reasonably be explained (Table III). A similar situation is found as regards CF transitions. Further work on this subject which requires a theoretical analysis of oscillator strengths in now under way. ACKNOWLEDGMENTS Thanks are due to Prof. Alexandro Bencini of University of Florence for providing us with a first version of the SCF-MS-Xα program and for fruitful discussions. This work has been supported by the CICYT (Project number PB 86-03041).


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