Low-temperature Fourier transform infrared spectra of tetrachlorozincate, tetrachloronickelate and tetrachlorocuprate with different counter ions

351

Vibrational Spectroscopy, 6 (1994) 351-362

Elsevier Science B.V., Amsterdam

Low-temperature Fourier transform infrared spectra of tetrachlorozincate, tetrachloronickelate and tetrachlorocuprate with different counter ions N. Trendafilova

and G.St. Nikolov

Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia (Bulgaria)

R. Kellner Institute of Analytical Chemistry, Technical Universi@ of Henna, 1060 Henna (Austria)

G. Bauer Institute of General Chemistry, Technical University of Menna, 1040 Vienna (Austria)

(Received 9th June 1993)

Abstract Room-temperature infrared (IR) and Raman and low-temperature (100 and 77 K) IR spectra of R,MCl, have been recorded in the 4000-50 cm- ’ frequency range. The metal ions Zn(II), Ni(I1) and Cu(II) form a series of increasing ionic radius, while the counter ions [Bu$Ij+, [Et4N]+, Rb+ and Cs+ reflect the order of decreasing bulkiness. According to the IR and Raman spectra, the [ML4]‘- units are a distorted tetrahedron, the distortion away from the tetrahedral structure increasing with the increasing size of the counter ion. Additional splittings due to the vibration-electronic coupling (Jahn-Teller coupling) was observed for the Cu(II) and Ni(I1) compounds. Keywords: Infrared spectrometry;

Raman spectrometry;

Tetrachlorometallates of the general formula R,MCl, (R = [Bu,N]+, [Et4N]+, Rb+, Cs+; M = Z&I), C&I) and NKII)] are of interest because of the occurrence of a large number of [MCl,]‘coordination polyhedra [ 11. For coordination number 4, both tetrahedral and near squareplanar geometries have been observed [2-101. Particular attention has been paid to the distortion of the [MCl,12- anion, away from the regular tetrahedral form to different intermediate Correspondence

to: N. Trendafilova, Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia (Bulgaria).

Counter ion; Jahn-Teller

coupling; Tetrachlorometallates

structures and to the factors that may influence the distortion. Vibrational spectroscopy is an important tool to deal with this specific problem [ll-291. The factors contributing to the observed geometries are: (1) Coupling of vibrational and electronic motions [Jahn-Teller (JT) coupling] was thought to be the major factor contributing to the distortion of many [CuC1,12- structures [23,24,27]. (2) The crystal packing requirements also influence the geometry of this class of compounds [28,29]. In fact in the absence of Jahn-Teller coupling it is the dominant factor. In the presence of the JT effect it helps or opposes the JT

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N. Trendafilova et al. / vib. Spectrosc. 6 (195’4)351-362

352

distortions. Recent study on the manifestation of the JT active modes in the vibrational spectra of Ni(I1) and Cu(I1) doped in [ZnCl,]*- has shown that the crystal packing effect of the host compound overrides the JT effect [24]. Crystal packing itself is mainly determined by the size of the counter ion [25]. (3) Hydrogen bonds being part of the crystal packing greatly affect the relative Cu-Cl bond length [25]. In general hydrogen bonds tend to remove charge from the Cl atom and thus reduce the electrostatic repulsion between Cl atoms and allow the [CuCl,]*- ion to move towards a square-planar geometry [S]. The strong intermolecular hydrogen bonding between the cation and the [CuCI,]*- anion was considered as the main reason for the distortion of the [CuCl,l*anions [lo]. By choosing the above series of compounds we intend to study (i) the effect of the counter ion size, which is manifested in the crystal packing conditions. The size decreases in the series [Bu,N]+, [Et4N]+, Rb+, Cs+; (ii) the effect of the electronic structure of the central ion. While Ni(I1) and especially Cu(I1) should exert JT distortions on their coordination polyhedron, the Zn(I1) ion is not prone to such distortion. Hence

Ion symmetry Td

Ion symmetry D2d

Fig. 1. Correlations for Cs,CuCl, [18].

in a comparative factors.

NORMAL COORDINATE ANALYSIS AND COMPUTATIONAL PROCEDURE

To help the assignment of the [MCl,]*- far-IR bands a normal coordinate analysis has been performed. The solid-state spectra at low temperature (77 K) of the studied compounds were used in the calculations. The spectra of [MCl,]*- anions [M = Zn(II), Ni(I1) and Cu(II)] were classified first into the four pure tetrahedral types of vibrations: symmetric stretch (v,), symmetric deformation (v2), asymmetric stretch (v3) and asymmetric deformation (v,). The nine normal modes of the anion in D,, ion symmetry belong to the representations (2a, + b, + 2b, + 2e) all of which are Raman active (see Figs. 1 and 2 and Refs. 2, 17 and 18). The tetrahedral Y*, Ye and V, modes split in D,, symmetry; v2 into (a, + b,) and both vg and V~ into (b, + e) (see Figs. 1 and 2 and Tables l-3). An attempt was made to classify the observed bands in the factor group symmetries [D,, for Cs,CuCl, and Rb,CuCl, and DJh for (Et,N),ZnCl,, (Et,N),NiCl, and (Et,N),C&l,]. No structural data were found for

Site symmetry CS

study we may trace these two

Factor group D2h

N. Trendafilova et al. /fib.

Ion

Spectrosc. 6 (1994) 351-362

Factor group

Site and Ion symmetry

symmetry

D4h

D2d

Td

353

Within the Born-Oppenheimer approximation, the vibrational secular determinant can be written as

IGF-AEI=~

Fig. 2. Correlations for (Et,N),MCI,

(M = Zn, Ni) [2,17,18].

(Et,N),CuCl, and all studied (Bu,N)-compounds and classification according to the factor group modes was not possible.

where G is the inverse kinetic energy matrix, F is the matrix describing potential energy and in. eludes the force constants, A is a matrix of erements 4~*$, where vi is an observed frequency, and E is the identity matrix. The normal modes of vibration were defined in terms of a mathematically complete set of internal coordinates for the anion. The vibrational secular determinant was solved using computer programs [30,311 with force constants taken from our previous study [24] and from Ref. 17. A valence force field was used. Five force constants were defined: one stretching, two bending and two constants for interactions between stretching and bending modes (for details see [24]).

TABLE 1 Far-infrared (room-temperature Cs,CuCl,

=

and 77 K) and Raman (room-temperature)

frequencies of Cs,CuCl,

Rb,CuCl,

a

300 K IR

77 K IR

300K Raman

300 K IR

77 K IR

300 K Raman

81s

82s

86w

82s

87s

88w

Lattice modes

96s

99s

-

v2 (eb) (Raman)

_

Assignment b Td

and Rb,CuCl,

D,,

(cm-‘)

D,,

bru, bs,

_

104w

104w

_

_

104w 1

a,&,J,

a,,b,(Raman)

_ 130m

120sh 131m 139m 152m

121w? 14Ow? -

129s _

_ 131s _

_ 147sh

ag or b2g ’

e(IR,Raman)

150sh 246m 255m

289m

249m,d 255s 262s _ 289s,d 300m

_

br, or bs, ’

v4 (t,b)

(IR,Raman)

-

172s

172s

-1

250m -

263s

269s

252s -

v3 0;)

b b:: or b,, ’

b,(IR,Raman)

br, or bs, ’ ax

b,(IR,Raman)

b b:I or b,,

e(IR,Raman)

b 3u

(IR,Raman) 266m -

-

_ 302m

315s -

286sh -1 315s

305s

’

d d

a s = strong, m = medium, w = weak, sh = shoulder, d = doublet, eb = e mode (bend), ti = t, mode (bend), ti = t2 mode (stretch). b The effective symmetry is always lower than Td. ’ In these cases the assignment is not unique and both alternatives are shown. d These bands could be combination modes; the frequency is too high to assign them as components of v3.

N. Trendafdova et al. / Vii. Spectrosc. 6 (I 994) 351-362

354 TABLE 2 Far-infrared (cm -1 ) a

(room-temperature

(Et,N)rZnCl,

and 77 K) and Raman (room-temperature) (Et,N),CuCl,

(Et,N),NiCl,

frequencies of (Et,N),MCl,

(M = Zn, Ni and Cu)

Assignment

b

300K IR

77K IR

300 K R

300K IR

77 K IR

300K Raman

300K IR

77 K IR

300K Raman

80m

74s

76m

77s

73w

75w

79w

81m

74w

D4h

D 2d

v2 (eb) (Raman)

b,J-) a,,(-)

b,(Raman) a,(Raman)

a,, + brg v4 (t,b)

eg + e,

e(IR,Raman) b,(IR,Raman)

e, + eg

e(IR,Raman)

b2e + a,,

b,(IR,Raman)

Tda

Lattice modes 97w

-

-

104w-

130 -

126s 130s -

278s

126s -

278sh 282s 290sh

276s -

-

-

-

94w

94s

-

-

-

-

-

-->

112s

119s

lllm 115m

117s 132sh -

122s 132sh 140s

115m 135m 1

298s

269sh 283s 3OOw

272s -

242sh 268s -

250s 268s 287s

276s I

305sh

-

-

_

106s

(IR,Raman)

v3

(ti)

(IR,Raman)

a

a See Table 1. b No structural data have been found and the assignment was done on the basis of comparison with (Et,N),ZnCl, and (Et,N),NiCl,.

TABLE 3 Far-infrared (room-temperature (Bu,N),ZnCl,

a

300 K

77 K

81W

79w

and 77 K) frequencies of (Bu4N),MC14 (M = Zn, Ni and Cu) (cm-‘)

(Bu4N),NiC14 a

(Bu,N)~C~CI,

a

Assignment b

300K

77 K

300K

77 K

Td

82w

82w

90m

86~

Lattice modes

-

103w

-

v2 (eb) (Raman)

D2d

85~

130s,d

131s 137sh -

113sh 122m 133sh 145sh

_

-

265sh

-

124s 133sh (149w 165m

124m

124s

lsOm -

lsOm -1

242s

225w

261m

264s

222w 233m 271s

YIkan) e(IR,Raman)

v4 (t,b)

(IR,Raman)

b,(IR,Raman) b,(IR,Raman)

v3

0;)

(IR,Raman) 279s

281s 290sh

290s

(28Osh) 290s

292s

296m1

e(IR,Raman)

300sh

300sh

-

300sh

314sh

-

b

a

Raman spectra of the (Bu,N),MCl,

were not possible to be measured. b See Table 1.

355

N. Trendafilova et al. / Kb. Spectrosc. 6 (1994) 351-362

in KBr, polyethylene disks and Nujol mulls in the 50-500 cm-’ range. The low-temperature FT-IR spectra were recorded with the same instrument using an attachment for low-temperature measurements at 300, (1701, 100 and 77 K. To record the low-frequency Raman spectrum, powdered samples were prepared as flat powder layers by weak pressure under metal plates. The (Et,N),NiCl, sample was enclosed in a capillary cell because of its hygroscopicity. The Raman

EXPERIMENTAL

The compounds were obtained and characterized according to the methods described previously in the literature: Cs,CuCl, and Rb,CuCl, in [16,18,19], (Et,N),MCl, (M = Zn, Cu, Ni) in [26] and (Bu,N),MCl, (M = Zn, Cu, Ni) in WI. All spectra were recorded with a Bruker IFS 113 V Fourier transform (ET) spectrometer. The room-temperature ET-IR spectra were recorded

Cs&uCL4.

(a)

40.0

4 450

400

350

300

200

150

100

6

200

150

100

50

250

WAVENUMBER

CM-I

300 250 WAVENUMBER

CM-1

(b)

46.25

35.0

4;

400

350

Fig. 3. Far-IR spectra of: (a) Cs2CuCl,

and (b) Rb,CuCl,

at different temperatures.

N. Trendafiova et al. / vib. Spectrosc. 6 (1994) 351-362

356

spectra of (Et,N)@Cl, were recorded with a RAMALOG 7 (SPEX, USA) double-beam spectrometer. The scattered light was being detected at a direction perpendicular to the excitation beam. A He-Ne laser (632.8 nm) was used to avoid luminescence which was observed under irradiation with green light. The Raman spectra of Rb,CuCl, and Cs,CuCl, were recorded with a RAMALOG spectrometer in a back scattering configuration mode. An argon ion laser (514.5 nm) was used for excitation. The power output was 300-600 mW. The crystalline powders were placed in a small glass-vessel with a thin flat bottom if hygroscopic or pellets were prepared. The resolution in all IR and Raman measurements was better than 3 cm-‘.

RESULTS AND DISCUSSION

The observed room- and low-temperature spectra will be discussed on the basis of the factor group correlation analysis [32,33] (Figs. 1 and 2) where structural data are available. Cs,CuCl, Cs,CuCl,.

and Rb,CuCl,

The crystal system is orthorhombic, the space group is P,,,, (DE>, with Z = 4. The anion has crystallographically imposed m symmetry and the three independent copper-chlorin: bond lengths are 2.244(4), 2.235(4) and 2.220(3) A [17]. The reported valence angle cz(ClCuCl) is 124”. The internal modes of vibration of the [MClJion at a site symmetry in the crystal can be correlated with the modes of isolated Td and D,, ions (Fig. 1) [16,18]. According to the selection rules for the crystal, the only Raman active Td modes, v1 and Ye, become IR active in the crystal. The bands are assigned in our study according to the D2,, factor group and the results are in line with those of Dunsmuir and Lane [l&19] and Beattie et al. [16]. The IR and Raman spectra of Cs,CuCl, at room temperature show that the V, mode splits into two components and v3 into three components (Table 1 and Fig. 3a); vr and v2, although allowed, were not observed. This

finding suggests that the symmetry is lower than Td at room temperature but the expected correlation splittings, according to Fig. 1, were strictly not observed. The low-temperature IR spectrum, however, shows additional splittings of the v3 and V, tetrahedral modes in agreement with the lower D,, symmetry (see Table 1). The vi and v2 IR components are symmetry allowed in D,, and they should occur most probably with low intensity in the vg and vq frequency ranges, respectively; it is therefore probable that they could be obscured by the more intense vg and v4 bands 1181. Rb,CuCI,. The room-temperature spectrum of Rb,CuCl, shows one band for v2 (eb) and v1 (@ each and two bands for the v, (t,b) (Fig. 3b). No additional splittings were observed in the lowtemperature spectrum. The differences in the band positions are over a range of only 2-5 cm-‘. Obviously, no changes occur in the [CuCl,]*structure when the temperature is lowered. The spectral pattern of the Rb,CuCl, vibrational spectrum differs strongly from that of Cs,CuCl,: the presence of two high-intensity components of vg, as well as the appearance of v2 in the IR spectrum suggest that the [CuCl,]*geometry is lower than D2d. The 172 cm- ’ component of vq in the IR spectrum is not present in any other [CuCl,]*--containing compound studied here. Its appearance at such a high frequency suggests that the [CuCl,]*- unit in the Rb,CuCl, may be closer to square-planar arrangement than in any other compounds; the [CuCl,]‘- units in this case adopt a two-dimensional layer structure, leading to a (4 + 2) octahedral coordination as suggested elsewhere [25]. (Et,N),MCl,

(M = Zn, Ni and Cu)

It has been reported that the (Et,N12ZnCld and (Et,Nj2NiCl, are isostructural, belonging to the space group P4,/nmc , with two molecules in the primitive unit cell (Z = 2) [26]. The internal vibrational modes of the [MCl,]*anions were derived by correlating the Td ion to D,, site symmetry and Ddh factor group (Fig. 2). The far-IR and Raman spectra of (Et,N),ZnCl, were assigned to the appropriate modes, predicted by a factor group analysis for the reported

h? Trendafilava

et al. /fib.

Spectrosc.

6 (1994) 351-362

357

space group. According to the correlations v1 and Ye are expected to be lacking in the IR spectrum [2]. The far-IR [room- and low-temperature (77

1. I

K)] and Raman (room temperature) spectra of (Et,N&MCl, are given in Table 2 and Fig. 4a-c. (Et,N),ZnCl,. The [ZnCl,]‘- room-temperature spectrum shows three strong well resolved

(ET~N)~ZNCL~

(4

tI

I

0.825

0..m WRYENUMBER

CM-1

250 WRVENUMBER

CM-I

0.825

450 Fig. 4. Far-IR

400

350

spectra of (a) (Et4N)2ZnC14,

300

(b) (Et,N),NiCl,

200

150

and (c) (Et,N),CuCl,

100

50

at different temperatures.

N. Trendafioua et al. /Vii.

Spectrosc. 6 (1994) 351-362

(CJ t

J

450

I 400

350

300

250 WRVENUMBER

200

150

I00

50

CM-1

Fig. 4 (continued).

bands near 80, 130 and 278 cm-‘. None of the predicted correlation splittings was observed at room temperature. On lowering the temperature the t, modes (130 and 278 cm-l, respectively) split only slightly (4 and 10 cm-‘, respectively) (see Table 2). The normal coordinate analysis of [ZnCl,l*- unit reproduced well the low-temperature (77 IQ spectrum with a very small deviation of the angles from their tetrahedral values. Thus, [ZnCl,]*- may be nearly tetrahedral both at room and low temperatures [24]. The Raman roomtemperature spectrum reproduces the room temperature IR spectrum with no shifts (Table 2). (Et,N),NiCl,. The features of the far-IR room-temperature spectrum of (Et,N),ZnCl, have been observed also for (Et,N),NiCl, (Fig. 4bl. The splittings predicted by the correlations between the ion site and factor group symmetry were not observed. Both vs and v4 modes split at 77 K, indicating the lower than Td symmetry of the [NiCl,]*unit. Since the room temperature IR and Raman spectra do not show the predicted correlation splittings, the additional splittings of the yg and v, modes, which occur in the low-temperature IR

spectra, indicate either a very weak JT coupling or that the JT coupling and the interligand repulsion factors act in opposite directions and they almost cancel out each other [24]. (Et,N),CuCl,. Since no structural data are available for (Et,N),CuCl,, the observed IR and Raman spectra will be discussed on the basis of the comparison with the observed spectra of the other two [Et,N]+ compounds of Ni and Zn and Cs,CuCl,. The room-temperature spectrum of [CuCl,l*- (Fig. 4c) shows that the symmetry of the unit is lower than tetrahedral even at room temperature, since the us (ts) and v, (tg) were already split into two well separated components. The v, (ti) mode splitting is 15 cm-’ and the us (tl) splitting is 26 cm-‘. At 77 K both v4 and us were split into three components spreading over a range of 18 and 37 cm-‘, respectively. One component of the degenerate mode stays unshifted while the other component is raised in frequency. Note that the lower-frequency non-degenerated mode is also shifted to higher frequency. This is typically a case of Jahn-Teller coupling when the totally symmetric mode produced in the lower symmetry remains unshifted

N. Trendafilova et al. / vib. Spectrosc. 6 (1994) 351-362

359

(BurN)zZhCL,

85.0

(a’

46.25

30.0 WAVENUMBER

CM-l (BUrN)zCuCL,

k) 90.0

450

400

350

300 250 WAVENUMBER

Fig. 5. Far-IR spectra of (a) (Bu.,N),ZnCl,,

200 CM-l

150

100

50

6) (Bu4N)2NiCl, and (c) (Bu4N),CuCl,

at different temperatures.

N. Trendafiloua et al. / vib. Spectrosc. 6 (I 994) 351-362

360

[24]. A comparison of Figs. 4a, b and c shows that in the case of (Et,N),CuCI, the observed splittings at room temperature and low temperature are larger than in the spectra of the other two compounds. Since the counter ion is the same, the observed larger splittings could be correlated and explained by static distortions determined by the Cu(I1) ion as suggested elsewhere [24,25].

COMPARISON OF (Et,N),CuCI, TRA

AND Cs,CuCl, SPEC-

The low-temperature splitting observed for the Ye (ti) mode in the Cs,CuCl, spectrum is different as compared with that in the (Et,N),CuCl, spectrum. The positions of three vg components did not change significantly (250 -+ 249, 268 + 262, 287 + 289 cm-‘) (Tables 1 and 21, but additional splitting was observed of the first and third components: (247,249 cm- ‘1 and (289,290 cm- ‘> for Cs,CuCl,. The far-IR spectrum of Cs,CuCl, shows a different spectral behaviour of the V, (t,b> tetrahedral mode as compared with (Et4Nj2CuCl,. At room temperature it splits into two components (130 cm-’ and a shoulder at 150 cm-‘). At low temperature the broad band at 130 cm-’ splits into two components (131 and 139 cm-‘) and a shoulder at 150 cm-’ becomes a medium intensity band. These differences can be explained in terms of the lower factor group for Cs,CuCl, as compared with that of (Et,N),CuCl,. (Bu, N),MCI, (M = Zn, Ni and Cu) - compatison with the (Et4 N),MCl, spectra

It was expected that the larger [Bu,Nl+ cation would produce larger splittings of the v3 and V, symmetry species in the room-temperature and low-temperature spectra as compared with the [Et,N]+ counter ion. Unfortunately no structural data are available for any of the studied [Bu,N]+ compounds and therefore detailed spectra-structure correlation is not possible. The bands were assigned on the basis of the Td-D,, correlations (Fig. 1). The far-IR (room temp. and low temp.1 spectra of (Bu,N),MCl, (M = Zn, Ni and Cu) are given in Table 3 and Fig. 5a-c.

(Bu, Nj2 Z&1,. The room-temperature and low-temperature spectra of (Bu,N),ZnCl, (Fig. 5a) show the same position of the V~ and V, modes as in the (Et,N),ZnCl, spectrum. The v’3 (t;) mode splits into the same number of bands but the splittings for (Bu,N),ZnCl, are larger (16 and 9 cm-’ with respect to the main peak at 281 cm-‘). These splittings were 4 and 8 cm-’ for (Et,N),ZnCl,. Obviously, the [ZnCl,]*- unit is nearly tetrahedral at room temperature for both compounds but at 77 K in (Bu,N),ZnCl, the [ZnCl,]*- unit deviates from the regular tetrahedron more strongly than in (Et,N),ZnCl,. While the splittings and shifts effected upon substitution of the counter ion can clearly be attributed to the size of this ion, it is not clear what causes the lower symmetry of the [ZnCl,l*- at 77 K. A factor that may play a leading role is intermolecular H-bonds as suggested elsewhere [8-101. Most certainly it is not related to the Jahn-Teller effect. Probably some termal averaging takes place at higher temperature that enhances the symmetry. (Bud N), NiCl,. The room-temperature spectrum of (Bu,N),NiCl, (Fig. 5b) shows a number of bands in addition to those observed in the (Et,Nj2NiCl, spectrum. The v3 0;) mode is not split at room temperature but V, (t,b> shows three components (shoulder 113, 122 and 133), which are split over a range of 10 cm- ‘. Namely the fact that the bending mode v4 (t,b) is split into three components in the (Bu,N),NiCl, room-temperature spectrum and this mode was not split in the (Et,N),NiCl, room-temperature spectrum, supports the suggestion that the larger counter ion [Bu,N]+ produces a more flattened structure than the smaller [EtqN]*+ counter ion. Further splittings have been observed when the temperature was lowered to 77 K. The main component of V~ (t,b) splits over a range of 41 cm-‘, the split components appearing at 124, 133 and 165 cm-’ (see Table 3). The stretching v3 (tg> mode splits into three components at 77 K. As compared with the splitting of this mode in (Et,N),NiCl, (A = 14, 17 cm-‘) the splitting here is 19 cm-‘, neglecting the shoulder at 280 cm-‘, which could be the missing v 1.

N. Trendafilova

et al. /Vii.

361

Spectrosc. 6 (1994) 351-362

The much larger splittings of V, (asymmetric deformation) as compared with those for the yj mode (asymmetric stretch) of the [NiCl,l*- unit in (Bu,N),NiCl, could be explained by large angular distortion. (Bu,N),CUCI,. The (Bu,N),CuCl, room-temperature spectrum displays that the symmetry of [CuCl,]*- is lower than Td even at room temperature since the two higher-frequency bands related to f, tetrahedral modes (vs and ~~1are split (Fig. 5~1. The spectral pattern in the room-temperature spectrum suggests that the [CuCl,]*symmetry is higher than C,, (vq is split into two components: 124 and 150 cm-‘, but v3 shows three components: 225, 264 and 292 cm-i>. The 77 K (Bu,N),CuCl, spectrum does not show any additional splitting of the v4 (t,b) band. A comparison between the 77 K spectra of (Et,N12CuCl, and (Bu,N),CuCl, shows a larger splitting of the v4 and especially of the yg stretching mode. The room-temperature splitting of the V~ (t,b) for (Et,N),CuCl, is 15 cm-’ and for (Bu,N),CuCl, it is 26 cm-‘. At the same time the splittings at 77 K of the v3 (t;) mode are 18 and 19 cm-’ for the first compound and 38 and 25 cm-’ for the second compound. Obviously the size of the [Bu,N]*- counter ion influences both angles cx and p resulting in significant changes of the V~(t,b) mode. Bond length variations are also expected as evidenced by the vg (ti) splitting. The normal coordinate analysis of [CuCl,]*reproduces the low-temperature (77 K) spectrum of (Bu,N),CuCl, with a larger variation of the opposite ClCuCl angles ((Y= 140”, p = 145”) as compared with their values for (Et,N12CuCl, (a = 125”, p = 130”). Higher values for the first bending force constant (F, = 0.80”) and lower for the second (F,, = 0.18) were needed in the calculations. Conclusions On the basis of the spectral behaviour of [ZnCl,]*-, [CuCl,]*- and [NiCl,]*- spectra it may be concluded that the number of the bands observed in the far IR spectra is determined by the static distortions which are connected with the size of the counter ion and the nature of the metal ion. The largest splittings of the t, modes

were observed in the spectra of the (Bu,N),MCl, compounds. The angular distortions towards more flattened structures of the [CuCl,]*- units are produced by both the counter ion and the intrinsic Jahn-Teller coupling, which act in the same direction. Cs+ produces approximately the same effect of splitting as the studied organic cations. The spectral pattern of Rb,CuCl, far IR spectrum has shown that Rb+ when acting as a counter ion in this class of compounds, probably defines a 4 + 2 coordination. This work was performed under the scientific technical agreement between Austria and Bulgaria. Financial support from the Austrian Ministry of Sciences and Research and from the Bulgarian Research Fund under Project 136 is greatly acknowledged. The authors are grateful to Dipl. Ing. M. Angelova, who prepared all the studied compounds. The low-temperature IR spectra of the investigated compounds have been recorded at Bruker by Dipl. Phys. G. Zachmann, to whom the authors express their gratitude.

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