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The vibrational spectra of alkali salts of hexahaloiridates
Spectroohtmics Acts,Vol.28A,pp.025to931. Pergamon Press 1072.Printed inNorthern Ireland
The vibrational speotra of alkali
salts
of hexahaloiridates
G. L. BOTTQER and A. E. SALWIN* Research Laboratories, Eestmsn Kodsk Company, Rochester, New York 14650 (Received7 Januaq 1971. Revtied 24 Aupt
1971)
Abstract-The Raman and i.r. spectra of alkali salts of octahhedrsl hexahaloiridstes have been measured at room temperature. Lattice vibrations and the stretching and bending frequencies of the iridium-halogen bonds were observed and interpreted. Force constant cslculations were used to obtain values for unobservable fundamental modes, and to compare covalency among various hexahalometalstes of the platinum metals. Certain dramatic differencesin the relative intensities of v1 and y2 in IrCls2-, IrBrs*- and IrIse- indicate that the supposed occurrenceof dynamic J&n-Teller effects may in some cases be quenchedby spin-orbit interactionand/or by charge transfer. ALTHOUGH numerous studies of the vibrational spectra of hexahalo complexes of various transition metals have been published, only fragmentary data exist concerning the spectra of the hexahaloiridates [l-9]. Certain anomalous features were found in the Raman spectra of the IrCl,2- ion [7], and they were attributed to a dynamic Jahn-Teller effect wherein the degenerate electronic ground state of the ion was coupled with vibrational modes of appropriate symmetry. Similar effects have been reported in Raman studies of ReCls2-, ReBrc2- and OsC1,2- [7, lo]. Since iridium forms octahedral hexahaloiridates in both the +3 (5-P) and the +4 (5@) oxidation states, these complexes offer an opportunity to study the relation of metal 5d orbital occupancy to possible dynamic Jahn-Teller effects. Moreover, instructive comparisons can be drawn between the spectra of the hexahaloiridate (III) ions and the well-documented spectra of the isoelectronic hexahaloplatinate (IV) ions [6,1 l-131 Hexahaloiridate ions are octahedral and are expected to have six vibrational modes. Of these modes, two F,, fundamentals are i.r. active (i.e. yg and y4) ; an A, (vr), an E, (Ye), and an F,, ( v5) fundamental are Raman active ; and an F,, (yg) *Eastman Kodak summer trainee, 1970. Present address: Department of Chemistry, Princeton University, Princeton, New Jersey 08540. [l] T. L. BROWN,W. G. McDuam, JR., and L. G. KENT, J. Am. Chem. Sm. 92, 3645 (1970). [2] D. M. ADAXS, Proc. Chews. Sot. 336 (1961). [3] D. M. ADAMS, J. C~~ATT,J. M. DAVIDSONand J. GERRATT,J. Chem Sot. (London) 2189 (1963). [4] D. M. ADAMSand H. A. GEBBIE,Spectrochim. Acta 19, 925 (1963). [5] M. LEPOSTELLOC, J. P. MATHIEUand H. POWLET,J. Chim. Phye. 60, 1319 (1963); G. PANNETIERand R. BONN-, J. Lese-C~-mnwn Metala, 18,411 (1969). [6] J. HIRAISHIand T. Sm OUCHI,Spectrochim. Acta %2, 1483 (1966). [7] P. J. HENDRAand P. J. D. PARK,Specbrochim. Acta aSA, 1636 (1967). [8] D. A. KELLY, Ph.D. Thesis, Louisiana State University (1969). [9] M. DEBEAU,Spectrochim. Acta %A, 1311 (1969); M. DEBEAUand H. POULET,Spectmchim. Acta 25A, 1563 (1969). [lo] L. A. WOODWARDmd M. J. WARE, Spectrochim. Acta Ro, 711 (1964). [ll] D. M. ADABB and D. M. MORRIS,J. Chem. Sot. A, 1666 (1967). [12] L. A. WOODWARDand J. A. CnxraaroN, Spectrochim. Acta 17, 594 (1961); L. A. WOODWARDand M. J. WARE, Spectrochim. Acta 19, 775 (1963). [13] I. R. BEATTIEand T. R. GILSON,Pvoc. Roy. Sot. Ser. A, 807,407 (1968); J. R. FERRARO, J. Chem. Phys. 55, 117 (1970). 926
926
G.
L. BOFCQERand A. E. SALWIN
fundamental is both i.r. and Raman inactive. The previous studies of the i.r. spectra of various crystalline compounds containing the IrCle2- ion [l-9] offer a reliable assignment of ys for this ion, but the assignment of va is not as certain. A similar situation exists in the assignment of the infrared spectrum of K,IrBr, [9]. Further, past work on the infrared spectra of crystalline compounds containing the IrCbaion demonstrated broad absorption bands because of the hydrates present, and consequently the resulting assignments are somewhat variable and uncertain. Apparently no vibrational spectra of any type have been published on hexaiodoiridate (IV). In the present work the infrared and Raman spectra of alkali salts of hexachloroiridate, hexabromoiridate and hexaiodoiridate have been examined. Both hexahaloiridate (III) and (IV) of chlorine and bromine were studied ; while only the hexaiodoiridate (IV) was investigated. The intense coloration of the hexahaloiridates presents no impediment to measuring their infrared spectra, but it does cause some difficulties in determining their Raman spectra. The most pronounced problems center on obtaining the Raman spectra of the hexahaloiridate (IV) complexes. These compounds are intensely black or blue-black, and have a substantial optical density throughout most of the visible region. Where possible we have measured all Raman spectra with a number of different laser lines so as to separate unequivocally the weak Raman scattering from laser emission spectra. EXPERIMENTAL Crystalline samples of K,IrCl,, K,IrCl,~n-H,O and K,IrI, + 30% KI were obtained from K + K Laboratories of New York. The Cs,IrCl, was prepared by dissolving K,IrCl, in a concentrated HCl solution and then adding to it an aqueous C&l solution. A water-free sample of K&Cl, was prepared in the Synthetic Chemical Division of the Eastman Kodak Company, and some of it was converted to AgJrCl, using standard procedures. The ilf21rBr, (where H = K, Rb and Cs) crystalline samples were prepared by a method analogous to that employed by others [l] to prepare Cs,SnBr,, with the additional feature of adding bromine to all of the solutions. The K.$rBr, salt was obtained by boiling K&Cl, in a concentrated HBr solution, and part of the solid compound was later converted to Ag81rBreby standard means. It was necessary to recrystallize the commercial K,IrI, sample to remove the large amount of alkali halide contaminant. Samples of K,OsCl, and K,OsBr, were prepared by previously outlined methods [l]. The i.r. spectra were investigated from 4000 to 300 cm-l with a Perkin-Elmer Model 521 Spectrometer to an accuracy of about f 1 cm-l. A modified PerkinElmer Model 301 Spectrometer [14] was employed to study the region from 500 to 20 cm-l at an accuracy of f 1 cm-l or better. All samples were studied in the i.r. at 300°K as Vaseline mulls supported on polyethylene or CsI substrates. The Raman spectra were recorded on a Spex Dual Monochromator Ramalog equipped with a Coherent Radiation Laboratories Model 52 Ar/Kr ion laser and with a Spectra Physics He/Ne ion laser. These lasers provided lines at 4880, 5145, 5682, 6328 and 6471 A. It was found that the focused laser lines impinging on the samples had to be kept at power levels below 10 mW to avoid sample decomposition. Narrow-band pass [ 141CJ.L. B~TTQEB, Speotrochim. Acta MA, 1821 (IQ08).
The vibrational spectra of alkali salts of hexahaloiridates
927
filters were used to remove laser emission lines from the spectra ; nevertheless, in some oases these lines were detected because of the extreme weakness of the Raman signal. The laser lines that gave the best results with each crystalline sample when using right-angle scattering geometry are as follows: K,IrCg and Cs&Cl,+3328 and 6471 A, K&C& and Ag,IrC+ all laser lines, H&Br,4880 A, KSIrBrs and Ag,IrBr,--6682 A, ILJrI,--6471 A and K,OsCl&5682 and 6471 A. No Reman spectra could be obtained for samples of K,OsBre with the right-angle scattering geometry when using the available laser lines; however, 180’ scattering gave the spectra tabulated in Table 2. All of the Raman spectra were taken with crystalline samples at 300°K with a spectral resolution of at least 6 cm-l and an accuracy of f 1 cm-l. Because of Rayleigh scattering from the samples, no meaningful Raman spectra could be obtained below 100 cm-l. The results of X-ray powder diffraction studies of the hexahalometalates (IV) used in this study show all except K,IrI, have the face-centered cubic K,PtCI, structure (Ohs). This is consistent with previous structural data on hexacbloro- and hexabromoiridate (IV) salts, and hexachloro- and hexabromoosmanate (IV) salts [lFi_171. Our X-ray data indicate that while KJrI,, K&C&, AgJrC&, KJrBrs and AgJrBr, are not rigorously isomorphous with KaPtCl,, they are structurally fairly similar to it. All samples used in this work were analyzed by spark-source mass spectroscopy and found to be of high purity. RESULTS The results of this study are presented in Figs. 1 and 2 and in Tables 1 and 2. Table 2 also contains frequencies reported elsewhere for a number of pertinent hexahalometalate anions in compounds isomorphous with K,PtC$. The conventional band notation [18] is used for the assignment of the fundamental frequencies. Although HENDRA and Pm indicate the presence of the combination bands y1 + va and va + vSin their i.r. data on K&C& and K,OsCl, [7], neither of these combination bands was found in this work. Indeed, K,OsBr, was the only sample in the current study in which i.r.-active combination bands were observed (resulting value of v1 is 226 cm-l and vais 169 cm-l). On the other hand, the Raman spectra of a number of the crystalline compounds showed evidence of the 2v, overtone similar to that found in the metal hexafluorides [19]. It should be mentioned that the Raman spectra of aqueous solutions of our compounds were generally similar to the solid-phase spectra. For example, in aqueous solutions of KJrBr,, v1 came at 200 cm-l and va came at 190 cm-l, and in aqueous solutions of K,IrB+, v1 was found at 212 cm-l and va at 178 cm-l. No evidence of chlorine or bromine isotopic fine structure was found in the solid state or solution vibrational spectra. This is because the slit widths we were forced to use (i.e. 2-6 cm-l) were larger than the expected isotopic splitting [2O].
[ 161 D. H. BROW, K. R. DIXON, C. M. LIVINUSTON,R. H. Nu-rru
and D. W. A. SUP, J. Chem. Sot. A, 100 (1967). [16] M. T. HUTCIEINOS and C. G. WINDSOR,Proc. Phye. Sot. 91, 928 (1967). [17] C. K. JORGENSEN, Acta C&m. Sea&. 17,1034 (1963). [ 181 See, for example,K. NAEAMOTO, InfraredSpectra of b&organic and coor&natiun Cornpow&, 2nd Edition. Wiley, New York (1970). [19] H.H.CL~~SEN,G.L.GOOD~,J.T.H~LL~~~~~~H.SE~U,J.C~~.P~~.~,~~~(~~~O). [ZO] W. Vm BRONS~YIC,R. J. H. CLaag and L. MARESCA,Inorg. C&m. 8,1396 (1969).
G. L. BOPNER and A. E. SALWIN ---r------T---
K2 Ir I,
400
300 900 100 Wavenumber, cm-’
Fig. 1. The i.r. spectra of crystalline potassium salts of hexahaloiridates. Sample temperature is 300’K.
Fig. 2. The Raman spectraof crystalline potassium salts of hexahaloiridates. Sample temperature is 300’K and asterisk denotes laser emission bands.
Table 1. Observed vibrational frequenciesof alkali salts of hexahaloiridatesat 300’K (cm-l) Infrared
Compound
Raman
VP
K,IrCl, Cs21rCls KsIrCl, AgW% K,IrBr, Rb21rBr, Cs,IrBr, KsIrBr, AgsIrBr, K,Ir&
v2
2%
%
333 321
182 182
87 72
362 341
-
-
188 177
(309 281 306 233 230 225 219 222 180
200 184 221 124 123 123 140 143 100
112
323
303
288
161
108 80 65 63 94 86 76
331 216 213 207 200 198 156
311 178 177 173 184 186 136
178 -
162 -
The vibrational spectra of alkali sa+ltaof hexahaloiridates
929
Table 2. Fundamental frequenciesof hexahalometaltte anions in solids of the type K.&X, (cm-l)*
oscls=1rC&aPa,+
352 362 3501
(274) t
(290) 320$
320 333 341$
176 182 186$
177 188 171s
OsBre2IrB+PtBr,S
218 216 217x
162 178 IQS$
227 233 243$
122 124
115f
78i
(125) (133) (121) (81)
* All unreferenced frequencies are from this work. Values in parentheses are calculated either from combination bands or from normal coordinate analysis. t Ref. 10. 2 Ref. 11.
The Raman spectrum of crystslline K,IrCl, shown in Fig. 2 compares fsvorably with HENDRA and Pm&s (HP) spectrum of that compound [7], yet our Raman spectrum of crystalline K,OsCl, (see Table 2) differs somewhat from both their spectrum and that of DEBEAU [9]. The Raman frequencies of crystalline K,OsCl, found by HP closely parallel the frequencies of the compound in aqueous solution [lo].The DEBEAU Raman spectrum of crystalline K,OsCl, indicated the presence of a V~band of considerable intensity, while both our spectrum and that of HP showed no measurable Raman intensity for Y%. The i.r. spectrum of KJrCl, shows minor absorption features on the shoulders of the Yeand vq fundamentals, and the i.r. spectrum of K,IrI, contains weak absorptions clearly due to the Reman-active v1 and vz fundamentals. The Rsman spectra of these compounds hsve no features anslogous to the observed infrared anomalies. It seems most likely that the snomalous infrared features are due to site or fs,ctor group effects, and that the hexshaloiridate ions are undistorted from octahedral symmetry in both K,IrCm, snd KJrI,. Estimates of the metal-halogen stretching force constants (fd) for the hexahalometallate anions were calculated from v1 snd v2 by the generalized force field approach outlined by PISTORIUS [21].These values, together with the force constant which reflects interaction between adjacent metal-halogen stretching motions (I&, are listed in Table 3. It should be noted that this calculation requires neglecting possible interactions between opposite metal-halogen stretching motions Cfaa’). In comparing a number of hexametalate metal-halogen force constants such an approximstion is necessary, since values for the v5 fundamentals have not been obtained in all samples and the number of calculated force constants cannot exceed the number of observed fundamentals. At any rate, it is expected that the derived force constant trends agree closely with those found using less approximate force fields [l]. The iridium-bromine stretching force constant calculated for K,IrBr, follows the trend set by the other hexabromometalate snions. The vibrational frequency data for K,IrCl, are insufficient to allow a value for their iridium-chlorine force constent to be rigorously derived; however, if one assumes the same trend of force constants in the [Zl] C. W. F. T.
PISTORIUS,
J. Chem. P&p. 29, 1328 (1968).
930
G. L.
BOXTUER and A. E. SALJVXN
Table 3. Force constants for hexahalometalatc anions (mdyn/A) Chloride
Bromide
Iodide
Anion
fa
faa
fa
faa
fa
f dd
OeX,~-
1.91 2.01 (2.07)* 2.28
0.17 0.04
1.67 1.69
0.17 0.05
-
-
1.73 1.93
0.12 0.07
IrX,% IrXs2ptx,z-
(0.13)* 0.07
1.53 -
0.07 -
* Deduced, see text.
hexachlorometalate anions as found in the hexabromometctlate anions, a value for fd of I&1,2- of approximately 2.07 mdyn/A is deduced. Thiaf, value leads to a frequency of v2 in IrCl,2- equal to 295 cm- l. The value of 295 cm-l from these results is considerably higher than the 225 cm-l value assigned by other workers on the basis of a presumed combination band [7]. Since our spectra show thst the v2 mode in hexabromoiridium compounds changes little in going from K21rBr6 to KJrBr,, we argue that the observed v2 at 303 cm-l in K&Cl, makes 295 cm-l a more plausible value for the v2 of K,IrCl, than 225 cm-l. Moreover, other workers have also calculated that v2should be higher than 225 cm-l [l], although their predictions were somewhat limited by a lack of self-consistent data. Certain comparisons can be made from a simple consider&ion of the force constant data in Ta;ble 3 : (i) The iridium-bromine force constant is greater in IrBr,2- thsn in IrBres-, no doubt because of the increased covelency with increased formal charge on iridium. It follows that the Ir-Br bond distance is greater in IrBres- than in IrBre2-, snd a like situ&ion exists concerning the iridium-chlorine bond distance in IrC1,3- and IrCl,2-. (ii) The force constants of the isoelectronic pair PtXe2- and IrXes- fall in the expected order [22], fd (PtXe2-) > fd (IrXe3-). (iii) Increasing d orbital occupancy in a series of hexahalometalates (IV) is clesrly relsted to increasing covalency.
We have obtained a resonable estimate of vs by assuming that the force constants of v6 and v6 are rtpproximately equal. This results in the relationship v6 = 0.707 vg, and values of vs so calculated are listed in Table 2. It has been demonstrated that such vslues of v,, are usually within 10 to 20 per cent of those calculated by more rigorous methods [20, 231. In the case of KJrCl, the agreement of the calculated value of v6 (i.e. 114 cm-l) and the value determined from the Ram&n-active overtone (i.e. 144 cm-l) falls essentially within this range of uncertainty. DISCUSSION
Crystals with the cubic K,PtCI, structure Frn3m (Ohs) have four molecules per unit cell. The PtCl,2- ions occupy sites of Oh symmetry and the cations occupy sites &l&z.?-ligandand RelatedV&rat&w, p. 49. St. Martin’s Press, New York (1968). [23] C. N. KRYNAUW and C. W. F. T. PISTORIUS, 2. Phys. Chem. New Folge 48, 113 (1964). [22] For example, D. M. ADAMS,
The vibrational spectra,of alkali s&s of hexahaloiridates
931
of T, symmetry [24].A group theoretical analysis of this crystal predicts that none of the internal modes of the Pt&*- ions will show splitting arising from the site or factor group effects. The observed spectra of compounds isomorphous with KzPtCl, agree with this prediction. Consideration of cation vibrations relative to the anion, and of anion rotations, predicts the occurrence of three lattice modes, one being i.r. active (F1,), one Raman active (PZ,), and one i.r. and Raman inactive (F,,). The i.r.active lattice vibration was observed in most compounds studied in this work but the other lattice modes were not seen. Certain changes in the relative intensities of r1 and r2 of the hexahaloiridate ions are shown in the Raman spectra in Fig. 2. Anomalies of this nature have previously been ascribed to the dynamic Jahn-Teller (J-T) effect [7,lo]. In this effect the time-averaged molecular structure is close to a regular octahedron but the molecule is visualized as oscillating between two forms of less than octahedral symmetry. Such an effect is theoretically possible when the ion exists in an orbitally degenerate electronic ground state, and when the degeneracy can be removed by vibrationelectronic coupling. Only two vibrational fundamentals (rz and r6) have the appropriate symmetry for such coupling. It is expected that the intensities and perhaps the frequencies of these two vibrations will be modified when dynamic J-T effects are present. Pt (IV) has the electronic structure 5de and its ground state in hexahaloplatinate complexes is not degenerate [25].Thus no J-T effect is expected in these ions and the relative intensity ratio of y1to Y%in solid samples (i.e. approximately one) [ll]is taken as the “normal” intensity ratio for 5d hexahalometalates. Since Ir(II1) is isoelectronic with Pt (IV), the intensity ratio of r1 to yp in hexahaloiridates (III) should also be about one. This prediction agrees with the observed Reman spectra of K,IrCl,, Ag,IrCl,, K,IrBr, and Ag31rBrs. Ir(IV), on the other hand, has a Sd6 electronic structure and its ground electronic state in hexahaloiridate (IV) is degenerate [25]. Because of electronic degeneracy these ions can experience the dynamic J-T effect and the intensity ratios of the Raman modes y1 to Ye might be expected to be anomalous. We observed such effects in the Raman spectra of crystalline K&Cl, and CszIrCl, but not in that of crystalline MZIrBr, (where N is K, Rb or Cs) or K,IrI,. It is known that dynamic J-T effects will not be displayed when the electronic degeneracy can be removed more easily by some other mechanism, such as spin-orbit coupling [26] or charge transfer [27]. Since the spin-orbit coupling of halogens decreases in the order I > Br > Cl [28], this coupling could stabilize K&I, and K,IrBr, against J-T distortions and yet be insufficient to quench the J-T distortion in K&Cl,. Likewise, on the basis of the halogen electronegativity trend (i.e. Cl > Br > I), charge transfer from halogen to metal could stabilize I&IrI, and KaIrBrs against J-T distortions but be too small to quench the J-T effect in K&Cl,. [24] R. W. G. WYCKOXT, Cq#al Structure, 2nd Edition, Vol. 3, p. 339. Interscience, New York (1966). [ZS] C. J. BAUJ.UUSEN, ht~uctio?a to lXgad Pi& Z%eoty. McGraw-Hill,New York (1902) [26] For example, F. 8. HAM, Phya. Reu. WA, 1727 (1966). [27] D. 8. McCL~, private communication. [28] D. 5. McC~uzz,J. C&m. Phye. 17,906(1949).
The vibrational speotra of alkali
salts
of hexahaloiridates
G. L. BOTTQER and A. E. SALWIN* Research Laboratories, Eestmsn Kodsk Company, Rochester, New York 14650 (Received7 Januaq 1971. Revtied 24 Aupt
1971)
Abstract-The Raman and i.r. spectra of alkali salts of octahhedrsl hexahaloiridstes have been measured at room temperature. Lattice vibrations and the stretching and bending frequencies of the iridium-halogen bonds were observed and interpreted. Force constant cslculations were used to obtain values for unobservable fundamental modes, and to compare covalency among various hexahalometalstes of the platinum metals. Certain dramatic differencesin the relative intensities of v1 and y2 in IrCls2-, IrBrs*- and IrIse- indicate that the supposed occurrenceof dynamic J&n-Teller effects may in some cases be quenchedby spin-orbit interactionand/or by charge transfer. ALTHOUGH numerous studies of the vibrational spectra of hexahalo complexes of various transition metals have been published, only fragmentary data exist concerning the spectra of the hexahaloiridates [l-9]. Certain anomalous features were found in the Raman spectra of the IrCl,2- ion [7], and they were attributed to a dynamic Jahn-Teller effect wherein the degenerate electronic ground state of the ion was coupled with vibrational modes of appropriate symmetry. Similar effects have been reported in Raman studies of ReCls2-, ReBrc2- and OsC1,2- [7, lo]. Since iridium forms octahedral hexahaloiridates in both the +3 (5-P) and the +4 (5@) oxidation states, these complexes offer an opportunity to study the relation of metal 5d orbital occupancy to possible dynamic Jahn-Teller effects. Moreover, instructive comparisons can be drawn between the spectra of the hexahaloiridate (III) ions and the well-documented spectra of the isoelectronic hexahaloplatinate (IV) ions [6,1 l-131 Hexahaloiridate ions are octahedral and are expected to have six vibrational modes. Of these modes, two F,, fundamentals are i.r. active (i.e. yg and y4) ; an A, (vr), an E, (Ye), and an F,, ( v5) fundamental are Raman active ; and an F,, (yg) *Eastman Kodak summer trainee, 1970. Present address: Department of Chemistry, Princeton University, Princeton, New Jersey 08540. [l] T. L. BROWN,W. G. McDuam, JR., and L. G. KENT, J. Am. Chem. Sm. 92, 3645 (1970). [2] D. M. ADAXS, Proc. Chews. Sot. 336 (1961). [3] D. M. ADAMS, J. C~~ATT,J. M. DAVIDSONand J. GERRATT,J. Chem Sot. (London) 2189 (1963). [4] D. M. ADAMSand H. A. GEBBIE,Spectrochim. Acta 19, 925 (1963). [5] M. LEPOSTELLOC, J. P. MATHIEUand H. POWLET,J. Chim. Phye. 60, 1319 (1963); G. PANNETIERand R. BONN-, J. Lese-C~-mnwn Metala, 18,411 (1969). [6] J. HIRAISHIand T. Sm OUCHI,Spectrochim. Acta %2, 1483 (1966). [7] P. J. HENDRAand P. J. D. PARK,Specbrochim. Acta aSA, 1636 (1967). [8] D. A. KELLY, Ph.D. Thesis, Louisiana State University (1969). [9] M. DEBEAU,Spectrochim. Acta %A, 1311 (1969); M. DEBEAUand H. POULET,Spectmchim. Acta 25A, 1563 (1969). [lo] L. A. WOODWARDmd M. J. WARE, Spectrochim. Acta Ro, 711 (1964). [ll] D. M. ADABB and D. M. MORRIS,J. Chem. Sot. A, 1666 (1967). [12] L. A. WOODWARDand J. A. CnxraaroN, Spectrochim. Acta 17, 594 (1961); L. A. WOODWARDand M. J. WARE, Spectrochim. Acta 19, 775 (1963). [13] I. R. BEATTIEand T. R. GILSON,Pvoc. Roy. Sot. Ser. A, 807,407 (1968); J. R. FERRARO, J. Chem. Phys. 55, 117 (1970). 926
926
G.
L. BOFCQERand A. E. SALWIN
fundamental is both i.r. and Raman inactive. The previous studies of the i.r. spectra of various crystalline compounds containing the IrCle2- ion [l-9] offer a reliable assignment of ys for this ion, but the assignment of va is not as certain. A similar situation exists in the assignment of the infrared spectrum of K,IrBr, [9]. Further, past work on the infrared spectra of crystalline compounds containing the IrCbaion demonstrated broad absorption bands because of the hydrates present, and consequently the resulting assignments are somewhat variable and uncertain. Apparently no vibrational spectra of any type have been published on hexaiodoiridate (IV). In the present work the infrared and Raman spectra of alkali salts of hexachloroiridate, hexabromoiridate and hexaiodoiridate have been examined. Both hexahaloiridate (III) and (IV) of chlorine and bromine were studied ; while only the hexaiodoiridate (IV) was investigated. The intense coloration of the hexahaloiridates presents no impediment to measuring their infrared spectra, but it does cause some difficulties in determining their Raman spectra. The most pronounced problems center on obtaining the Raman spectra of the hexahaloiridate (IV) complexes. These compounds are intensely black or blue-black, and have a substantial optical density throughout most of the visible region. Where possible we have measured all Raman spectra with a number of different laser lines so as to separate unequivocally the weak Raman scattering from laser emission spectra. EXPERIMENTAL Crystalline samples of K,IrCl,, K,IrCl,~n-H,O and K,IrI, + 30% KI were obtained from K + K Laboratories of New York. The Cs,IrCl, was prepared by dissolving K,IrCl, in a concentrated HCl solution and then adding to it an aqueous C&l solution. A water-free sample of K&Cl, was prepared in the Synthetic Chemical Division of the Eastman Kodak Company, and some of it was converted to AgJrCl, using standard procedures. The ilf21rBr, (where H = K, Rb and Cs) crystalline samples were prepared by a method analogous to that employed by others [l] to prepare Cs,SnBr,, with the additional feature of adding bromine to all of the solutions. The K.$rBr, salt was obtained by boiling K&Cl, in a concentrated HBr solution, and part of the solid compound was later converted to Ag81rBreby standard means. It was necessary to recrystallize the commercial K,IrI, sample to remove the large amount of alkali halide contaminant. Samples of K,OsCl, and K,OsBr, were prepared by previously outlined methods [l]. The i.r. spectra were investigated from 4000 to 300 cm-l with a Perkin-Elmer Model 521 Spectrometer to an accuracy of about f 1 cm-l. A modified PerkinElmer Model 301 Spectrometer [14] was employed to study the region from 500 to 20 cm-l at an accuracy of f 1 cm-l or better. All samples were studied in the i.r. at 300°K as Vaseline mulls supported on polyethylene or CsI substrates. The Raman spectra were recorded on a Spex Dual Monochromator Ramalog equipped with a Coherent Radiation Laboratories Model 52 Ar/Kr ion laser and with a Spectra Physics He/Ne ion laser. These lasers provided lines at 4880, 5145, 5682, 6328 and 6471 A. It was found that the focused laser lines impinging on the samples had to be kept at power levels below 10 mW to avoid sample decomposition. Narrow-band pass [ 141CJ.L. B~TTQEB, Speotrochim. Acta MA, 1821 (IQ08).
The vibrational spectra of alkali salts of hexahaloiridates
927
filters were used to remove laser emission lines from the spectra ; nevertheless, in some oases these lines were detected because of the extreme weakness of the Raman signal. The laser lines that gave the best results with each crystalline sample when using right-angle scattering geometry are as follows: K,IrCg and Cs&Cl,+3328 and 6471 A, K&C& and Ag,IrC+ all laser lines, H&Br,4880 A, KSIrBrs and Ag,IrBr,--6682 A, ILJrI,--6471 A and K,OsCl&5682 and 6471 A. No Reman spectra could be obtained for samples of K,OsBre with the right-angle scattering geometry when using the available laser lines; however, 180’ scattering gave the spectra tabulated in Table 2. All of the Raman spectra were taken with crystalline samples at 300°K with a spectral resolution of at least 6 cm-l and an accuracy of f 1 cm-l. Because of Rayleigh scattering from the samples, no meaningful Raman spectra could be obtained below 100 cm-l. The results of X-ray powder diffraction studies of the hexahalometalates (IV) used in this study show all except K,IrI, have the face-centered cubic K,PtCI, structure (Ohs). This is consistent with previous structural data on hexacbloro- and hexabromoiridate (IV) salts, and hexachloro- and hexabromoosmanate (IV) salts [lFi_171. Our X-ray data indicate that while KJrI,, K&C&, AgJrC&, KJrBrs and AgJrBr, are not rigorously isomorphous with KaPtCl,, they are structurally fairly similar to it. All samples used in this work were analyzed by spark-source mass spectroscopy and found to be of high purity. RESULTS The results of this study are presented in Figs. 1 and 2 and in Tables 1 and 2. Table 2 also contains frequencies reported elsewhere for a number of pertinent hexahalometalate anions in compounds isomorphous with K,PtC$. The conventional band notation [18] is used for the assignment of the fundamental frequencies. Although HENDRA and Pm indicate the presence of the combination bands y1 + va and va + vSin their i.r. data on K&C& and K,OsCl, [7], neither of these combination bands was found in this work. Indeed, K,OsBr, was the only sample in the current study in which i.r.-active combination bands were observed (resulting value of v1 is 226 cm-l and vais 169 cm-l). On the other hand, the Raman spectra of a number of the crystalline compounds showed evidence of the 2v, overtone similar to that found in the metal hexafluorides [19]. It should be mentioned that the Raman spectra of aqueous solutions of our compounds were generally similar to the solid-phase spectra. For example, in aqueous solutions of KJrBr,, v1 came at 200 cm-l and va came at 190 cm-l, and in aqueous solutions of K,IrB+, v1 was found at 212 cm-l and va at 178 cm-l. No evidence of chlorine or bromine isotopic fine structure was found in the solid state or solution vibrational spectra. This is because the slit widths we were forced to use (i.e. 2-6 cm-l) were larger than the expected isotopic splitting [2O].
[ 161 D. H. BROW, K. R. DIXON, C. M. LIVINUSTON,R. H. Nu-rru
and D. W. A. SUP, J. Chem. Sot. A, 100 (1967). [16] M. T. HUTCIEINOS and C. G. WINDSOR,Proc. Phye. Sot. 91, 928 (1967). [17] C. K. JORGENSEN, Acta C&m. Sea&. 17,1034 (1963). [ 181 See, for example,K. NAEAMOTO, InfraredSpectra of b&organic and coor&natiun Cornpow&, 2nd Edition. Wiley, New York (1970). [19] H.H.CL~~SEN,G.L.GOOD~,J.T.H~LL~~~~~~H.SE~U,J.C~~.P~~.~,~~~(~~~O). [ZO] W. Vm BRONS~YIC,R. J. H. CLaag and L. MARESCA,Inorg. C&m. 8,1396 (1969).
G. L. BOPNER and A. E. SALWIN ---r------T---
K2 Ir I,
400
300 900 100 Wavenumber, cm-’
Fig. 1. The i.r. spectra of crystalline potassium salts of hexahaloiridates. Sample temperature is 300’K.
Fig. 2. The Raman spectraof crystalline potassium salts of hexahaloiridates. Sample temperature is 300’K and asterisk denotes laser emission bands.
Table 1. Observed vibrational frequenciesof alkali salts of hexahaloiridatesat 300’K (cm-l) Infrared
Compound
Raman
VP
K,IrCl, Cs21rCls KsIrCl, AgW% K,IrBr, Rb21rBr, Cs,IrBr, KsIrBr, AgsIrBr, K,Ir&
v2
2%
%
333 321
182 182
87 72
362 341
-
-
188 177
(309 281 306 233 230 225 219 222 180
200 184 221 124 123 123 140 143 100
112
323
303
288
161
108 80 65 63 94 86 76
331 216 213 207 200 198 156
311 178 177 173 184 186 136
178 -
162 -
The vibrational spectra of alkali sa+ltaof hexahaloiridates
929
Table 2. Fundamental frequenciesof hexahalometaltte anions in solids of the type K.&X, (cm-l)*
oscls=1rC&aPa,+
352 362 3501
(274) t
(290) 320$
320 333 341$
176 182 186$
177 188 171s
OsBre2IrB+PtBr,S
218 216 217x
162 178 IQS$
227 233 243$
122 124
115f
78i
(125) (133) (121) (81)
* All unreferenced frequencies are from this work. Values in parentheses are calculated either from combination bands or from normal coordinate analysis. t Ref. 10. 2 Ref. 11.
The Raman spectrum of crystslline K,IrCl, shown in Fig. 2 compares fsvorably with HENDRA and Pm&s (HP) spectrum of that compound [7], yet our Raman spectrum of crystalline K,OsCl, (see Table 2) differs somewhat from both their spectrum and that of DEBEAU [9]. The Raman frequencies of crystalline K,OsCl, found by HP closely parallel the frequencies of the compound in aqueous solution [lo].The DEBEAU Raman spectrum of crystalline K,OsCl, indicated the presence of a V~band of considerable intensity, while both our spectrum and that of HP showed no measurable Raman intensity for Y%. The i.r. spectrum of KJrCl, shows minor absorption features on the shoulders of the Yeand vq fundamentals, and the i.r. spectrum of K,IrI, contains weak absorptions clearly due to the Reman-active v1 and vz fundamentals. The Rsman spectra of these compounds hsve no features anslogous to the observed infrared anomalies. It seems most likely that the snomalous infrared features are due to site or fs,ctor group effects, and that the hexshaloiridate ions are undistorted from octahedral symmetry in both K,IrCm, snd KJrI,. Estimates of the metal-halogen stretching force constants (fd) for the hexahalometallate anions were calculated from v1 snd v2 by the generalized force field approach outlined by PISTORIUS [21].These values, together with the force constant which reflects interaction between adjacent metal-halogen stretching motions (I&, are listed in Table 3. It should be noted that this calculation requires neglecting possible interactions between opposite metal-halogen stretching motions Cfaa’). In comparing a number of hexametalate metal-halogen force constants such an approximstion is necessary, since values for the v5 fundamentals have not been obtained in all samples and the number of calculated force constants cannot exceed the number of observed fundamentals. At any rate, it is expected that the derived force constant trends agree closely with those found using less approximate force fields [l]. The iridium-bromine stretching force constant calculated for K,IrBr, follows the trend set by the other hexabromometalate snions. The vibrational frequency data for K,IrCl, are insufficient to allow a value for their iridium-chlorine force constent to be rigorously derived; however, if one assumes the same trend of force constants in the [Zl] C. W. F. T.
PISTORIUS,
J. Chem. P&p. 29, 1328 (1968).
930
G. L.
BOXTUER and A. E. SALJVXN
Table 3. Force constants for hexahalometalatc anions (mdyn/A) Chloride
Bromide
Iodide
Anion
fa
faa
fa
faa
fa
f dd
OeX,~-
1.91 2.01 (2.07)* 2.28
0.17 0.04
1.67 1.69
0.17 0.05
-
-
1.73 1.93
0.12 0.07
IrX,% IrXs2ptx,z-
(0.13)* 0.07
1.53 -
0.07 -
* Deduced, see text.
hexachlorometalate anions as found in the hexabromometctlate anions, a value for fd of I&1,2- of approximately 2.07 mdyn/A is deduced. Thiaf, value leads to a frequency of v2 in IrCl,2- equal to 295 cm- l. The value of 295 cm-l from these results is considerably higher than the 225 cm-l value assigned by other workers on the basis of a presumed combination band [7]. Since our spectra show thst the v2 mode in hexabromoiridium compounds changes little in going from K21rBr6 to KJrBr,, we argue that the observed v2 at 303 cm-l in K&Cl, makes 295 cm-l a more plausible value for the v2 of K,IrCl, than 225 cm-l. Moreover, other workers have also calculated that v2should be higher than 225 cm-l [l], although their predictions were somewhat limited by a lack of self-consistent data. Certain comparisons can be made from a simple consider&ion of the force constant data in Ta;ble 3 : (i) The iridium-bromine force constant is greater in IrBr,2- thsn in IrBres-, no doubt because of the increased covelency with increased formal charge on iridium. It follows that the Ir-Br bond distance is greater in IrBres- than in IrBre2-, snd a like situ&ion exists concerning the iridium-chlorine bond distance in IrC1,3- and IrCl,2-. (ii) The force constants of the isoelectronic pair PtXe2- and IrXes- fall in the expected order [22], fd (PtXe2-) > fd (IrXe3-). (iii) Increasing d orbital occupancy in a series of hexahalometalates (IV) is clesrly relsted to increasing covalency.
We have obtained a resonable estimate of vs by assuming that the force constants of v6 and v6 are rtpproximately equal. This results in the relationship v6 = 0.707 vg, and values of vs so calculated are listed in Table 2. It has been demonstrated that such vslues of v,, are usually within 10 to 20 per cent of those calculated by more rigorous methods [20, 231. In the case of KJrCl, the agreement of the calculated value of v6 (i.e. 114 cm-l) and the value determined from the Ram&n-active overtone (i.e. 144 cm-l) falls essentially within this range of uncertainty. DISCUSSION
Crystals with the cubic K,PtCI, structure Frn3m (Ohs) have four molecules per unit cell. The PtCl,2- ions occupy sites of Oh symmetry and the cations occupy sites &l&z.?-ligandand RelatedV&rat&w, p. 49. St. Martin’s Press, New York (1968). [23] C. N. KRYNAUW and C. W. F. T. PISTORIUS, 2. Phys. Chem. New Folge 48, 113 (1964). [22] For example, D. M. ADAMS,
The vibrational spectra,of alkali s&s of hexahaloiridates
931
of T, symmetry [24].A group theoretical analysis of this crystal predicts that none of the internal modes of the Pt&*- ions will show splitting arising from the site or factor group effects. The observed spectra of compounds isomorphous with KzPtCl, agree with this prediction. Consideration of cation vibrations relative to the anion, and of anion rotations, predicts the occurrence of three lattice modes, one being i.r. active (F1,), one Raman active (PZ,), and one i.r. and Raman inactive (F,,). The i.r.active lattice vibration was observed in most compounds studied in this work but the other lattice modes were not seen. Certain changes in the relative intensities of r1 and r2 of the hexahaloiridate ions are shown in the Raman spectra in Fig. 2. Anomalies of this nature have previously been ascribed to the dynamic Jahn-Teller (J-T) effect [7,lo]. In this effect the time-averaged molecular structure is close to a regular octahedron but the molecule is visualized as oscillating between two forms of less than octahedral symmetry. Such an effect is theoretically possible when the ion exists in an orbitally degenerate electronic ground state, and when the degeneracy can be removed by vibrationelectronic coupling. Only two vibrational fundamentals (rz and r6) have the appropriate symmetry for such coupling. It is expected that the intensities and perhaps the frequencies of these two vibrations will be modified when dynamic J-T effects are present. Pt (IV) has the electronic structure 5de and its ground state in hexahaloplatinate complexes is not degenerate [25].Thus no J-T effect is expected in these ions and the relative intensity ratio of y1to Y%in solid samples (i.e. approximately one) [ll]is taken as the “normal” intensity ratio for 5d hexahalometalates. Since Ir(II1) is isoelectronic with Pt (IV), the intensity ratio of r1 to yp in hexahaloiridates (III) should also be about one. This prediction agrees with the observed Reman spectra of K,IrCl,, Ag,IrCl,, K,IrBr, and Ag31rBrs. Ir(IV), on the other hand, has a Sd6 electronic structure and its ground electronic state in hexahaloiridate (IV) is degenerate [25]. Because of electronic degeneracy these ions can experience the dynamic J-T effect and the intensity ratios of the Raman modes y1 to Ye might be expected to be anomalous. We observed such effects in the Raman spectra of crystalline K&Cl, and CszIrCl, but not in that of crystalline MZIrBr, (where N is K, Rb or Cs) or K,IrI,. It is known that dynamic J-T effects will not be displayed when the electronic degeneracy can be removed more easily by some other mechanism, such as spin-orbit coupling [26] or charge transfer [27]. Since the spin-orbit coupling of halogens decreases in the order I > Br > Cl [28], this coupling could stabilize K&I, and K,IrBr, against J-T distortions and yet be insufficient to quench the J-T distortion in K&Cl,. Likewise, on the basis of the halogen electronegativity trend (i.e. Cl > Br > I), charge transfer from halogen to metal could stabilize I&IrI, and KaIrBrs against J-T distortions but be too small to quench the J-T effect in K&Cl,. [24] R. W. G. WYCKOXT, Cq#al Structure, 2nd Edition, Vol. 3, p. 339. Interscience, New York (1966). [ZS] C. J. BAUJ.UUSEN, ht~uctio?a to lXgad Pi& Z%eoty. McGraw-Hill,New York (1902) [26] For example, F. 8. HAM, Phya. Reu. WA, 1727 (1966). [27] D. 8. McCL~, private communication. [28] D. 5. McC~uzz,J. C&m. Phye. 17,906(1949).