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Vibrational spectra of niobocene dichloride
Vibrational Spectroscopy 14 Ž1997. 147–150
Short Communication
Vibrational spectra of niobocene dichloride ˇ J. Holubova´ ) , Z. Cernosek, ˇ I. Pavlık ´ Department of General and Inorganic Chemistry, Faculty of Chemical Technology, UniÕersity Pardubice, nam.Legiı ´ ´ 565, CZ-532 10 Pardubice, Czech Republic Received 29 August 1996; accepted 16 October 1996
Abstract The infrared and Raman spectra of niobocene dichloride were recorded and briefly discussed. The results were compared with the infrared spectra of niobocene di-iodide. The assignment of both infrared and Raman bands was proposed. Keywords: Raman spectroscopy; Niobocene dichloride; Niobocene di-iodide
1. Introduction Niobocene dichloride ŽŽh 5-C 5 H 5 . 2 NbCl 2 . belongs to the group of so called bent metallocenes. The structure of this compound was determined by means of X-ray diffractometry and can be described as C 2v point group symmetry w1x. The object of this article is to study the IR and Raman spectra of niobocene dichloride especially in the far infrared region Ž500–100 cmy1 .. In this low frequency region the spectral bands belong to niobium–cyclopentadienyl ring Žhereafter Nb–Cp. and niobium–chlorine Žhereafter Nb–Cl. bond vibrations. Assuming the cyclopentadienyl rings are rigid disks the compounds approximately have C 2v symmetry and the expected skeletal vibrations are divided as follows: 5A 1 q 3A 2 q 3B1 q 4B 2 . In C 2v symmetry A 1 , B 1 and B 2 type of vibrations are both IR and Raman active, while the A 2 modes are only Raman active. The interpretation of the far IR spectrum is complicated because Nb–Cp and Nb–Cl )
Corresponding author.
stretching vibrations occur in the same spectral region. In order to distinguish between these two types of vibrations we studied the IR spectrum of niobocene di-iodide. The vibrational spectra of title compound in the region 3200–500 cmy1 will be also briefly discussed. In this spectral region we expect intra-ring vibrational modes associated with motion within the C 5 H 5 rings. In local symmetry C 5v the expected fundamental vibrations are 3A 1 q A 2 q 4E 1 q 6E 2 . The A 1 and E 1 vibrational modes are IR and Raman active whereas the E 2 vibrations are only Raman active. 2. Experimental The niobocene dichloride used in our study was a commercial compound ŽFluka. of guarantied purity better than 97%. The same compound was employed for preparation and purification of niobocene diiodide by the method of Hunter and co-workers w2x. The purity of niobocene di-iodide was examined by means of EPR spectroscopy.
0924-2031r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 4 - 2 0 3 1 Ž 9 6 . 0 0 0 6 2 - 8
148
J. HoluboÕa´ et al.r Vibrational Spectroscopy 14 (1997) 147–150
Transmissional IR spectra were recorded at 1 cmy1 resolution on Perkin-Elmer 684 as nujol mulls between KBr plates with a KBr reference in the region 3200–450 cmy1 and in polyethylene disk in the region 450–200 cmy1 . Kubelka–Munk reflection spectra were measured at 2 cmy1 on FT-IR spectrometer Bio-Rad FTS 45 using KBr in mid-infrared and polyethylene in far-infrared region Ž250 scans were averaged.. Raman spectra of microcrystalline samples were recorded in back scattering geometry on FT spectrometer Bruker IFS-55 with FRA 106 FT Raman equipment using a diode pumped Nd:YAG laser Ž1064 nm. and a nitrogen cooled Ge detector. Microcrystalline powder was placed in a cavity of an aluminium target. The power of the incident light was 25 mWrmm2 , the resolution was 2 cmy1 ; 250 scans were averaged.
Table 1 Proposed assignment for the vibrational spectra of niobocene dichloride and niobocene di-iodide in the intra-ring vibrations range Cp 2 NbCl 2 Ž n Žcmy1 ..
Cp 2 NbI 2 Ž n Žcmy1 ..
Assignment
infrared
Local symmetry ŽC 5v .
Raman
infrared
3120m 3092sh 1449m 1371w — 1125vs 1080m 1067m 855vw 834w 602vw
3097m 3093sh 1440m 1380m 1180vw 1126w 1077vw 1012m 853w 820vs 602vw
3097m — 1447m 1375m 1180vw 1142w 1080vw 1015m 860w 817vs 626vw
A1 E1 E1 E2 E2 A1 E2 E1 E1 A1 E2
nsŽCH. naŽCH. naŽCC. naŽCC. d ŽCH. nsŽCC. g ŽCH. d ŽCH. g ŽCH. g ŽCH. g ŽCC.
n : stretching, d : symmetric out-of-plane deformation, g : asymmetric out-of-plane deformation.
3. Results and discussion 3.1. Infrared spectrum The characteristic bands of fundamental vibrations of cyclopentadienyl rings appear in region
Fig. 1. Infrared spectra of ŽA. niobocene dichloride and ŽB. niobocene di-iodide in the range of intra-ring modes. Shaded area in spectrum ŽA. indicates skeletal niobium-ligands vibrations.
4000–500 cmy1 w3x, Fig. 1. The similarities of the spectra to those of sandwich compounds Cp 2 Fe a Cp 2 Ru w4x and to spectra of related bent metallocenes Cp 2 MX 2 ŽM s Ti, Zr, Hf, V and X s Cl, Br, I. of which the vibrational analyses are available in literature w5–9x allow a simple assignment. In the IR spectrum of niobocene dichloride appear seven active fundamental vibrations which belong to symmetry A 1 and E 1 , Table 1. In spectrum, Fig. 1, we unambiguously identified absorption bands at 3097, 1440, 1126, 1012 and 820 cmy1 . The presence of these bands confirms h 5-coordination of the Cp ring. We deduced from the 1126 cmy1 band that the Cp ring is not substituted w10x. In the spectrum of this compound four very weak bands ŽE 2 . were found, that are forbidden under C 5v symmetry. In the discussed spectral region the spectrum of niobocene di-iodide is identical with the above mentioned spectrum. As expected the change of halogen ligands does significantly not affect the energy of Cp ring vibrations. This result agrees with similar studies of vanadocene dihalides w10x. Table 1 shows the proposed assignment of bands. Energies of skeletal Nb–halogen and Nb–Cp vibrations appear in the range of 500–100 cmy1 . Twelve skeletal modes Ž5A 1 q 3B1 q 4B 2 . are allowed in IR spectra. Comparing the spectra of both
J. HoluboÕa´ et al.r Vibrational Spectroscopy 14 (1997) 147–150
Fig. 2. FT-infrared spectrum of niobocene dichloride.
compounds ŽFigs. 2 and 3. resolves two groups of absorption bands: Ž1. Several bands at 400–375 cmy1 and one band at 303 cmy1 ; exchange of halogen ligands does not effect their energies. Ž2. Several bands at 300–260 cmy1 ; chlorine to iodine ligand exchange shifts the band energies toward low values. On the basis of these results we assume the first group of absorption bands is assigned to stretching
149
vibrations of Nb–Cp and the second one to stretching vibrations of Nb–halogen. On the basis of previously obtained data for some other metallocene dihalides w7,9,10x middle intense bands at 389 and 303 cmy1 in the niobocene dichloride spectrum can be assigned to naŽNb–Cp. of B 2 symmetry and nsŽNb–Cp. of A 1 symmetry. The same conclusion can be drawn for bands of niobocene di-iodide at 392 and 297 cmy1 . Middle intense bands of niobocene dichloride at 377 and 353 cmy1 , the position that does not change with exchange of halogen ligands, could be classified as ring-tilt vibration t ŽCp. with agreement to literature w10x. Strong bands at 289 and 266 cmy1 are related to naŽNb–Cl. of B 1 symmetry and nsŽNb–Cl. of A 1 symmetry and very strong bands in spectrum of niobocene di-iodide at 143 and 125 cmy1 to corresponding vibration of Nb–I bond. This assignment agrees with the observation that the ratio n ŽM-y I.rn ŽM-yCl. ; 0.62 for many tetra- and hexacoordinated complexes w10–14x. The middle intense band at 176 cmy1 in the niobocene dichloride spectrum Ž169 cmy1 in the niobocene di-iodide spectrum. corresponds to a deformation vibration Cp–Nb–Cp in agreement to analyses of IR spectra of vanadocene dihalides w10x.
Table 2 Proposed assignment for the vibrational spectra of niobocene dichloride and niobocene di-iodide in the range of vibrations of metal-ligand bonds
Fig. 3. FT-infrared spectrum of niobocene di-iodide.
Cp 2 NbCl 2 Ž n Žcmy1 ..
Cp 2 NbI 2 Ž n Žcmy1 ..
Raman
infrared
infrared
390m 380sh — 314sh 308vs 288m 270sh 226w 190sh 179m 149m — —
389m 377m 353m — 303s 289vs 266vs 228w 188m 176m 150m — —
392m 380m 353m — 297m — — 206vw — 169m — 143vs 125vs
Local symmetry ŽC 2v .
Assignment
B2 B1 B2
naŽNbyCp. t ŽCp. t ŽCp.
A1 B1 A1 A1 B1 B2 A1 B1 A1
nsŽNbyCp. naŽNbyCl. nsŽNbyCl. t ŽCp. daŽCl–Nb–Cl. daŽCp–Nb–Cp. dsŽCl–Nb–Cl. naŽNb-I. nsŽNb-I.
n : stretching, d : bending, t: tilting.
J. HoluboÕa´ et al.r Vibrational Spectroscopy 14 (1997) 147–150
150
bands at 390 cmy1 and 380 cmy1 can be classified as stretching Nb–Cp bond vibrations as well as a very strong band at 308 cmy1 . The latter band was identified as vibration nsŽNb–Cp. of A 1 symmetry in agreement to several studies w5,6x. In the case of metal–halogen vibrations the authors w7,8x present vibrations of bent metallocenes in Raman spectra which are less intensive as in IR spectra. This conclusion agrees with our observation. The proposed assignment of Raman active skeletal vibrations is given in Table 2.
Fig. 4. FT-Raman spectrum of niobocene dichloride.
The proposed assignment of IR active bands is shown in Table 2. 3.2. Raman spectrum Fig. 4 displays the Raman spectrum of niobocene dichloride, 10 intra-ring vibrations were identified in the spectral region of 3200–500 cmy1 described in Table 1. Detail of Raman spectrum in the spectral region 500–100 cmy1 and its computer deconvolution is presented in Fig. 5. In our opinion middle intense
Fig. 5. Detail of FT-Raman spectrum of niobocene dichloride and its computer deconvolution.
Acknowledgements We would like to thank Dr. V. Smreka ` and Dr. A. Vidourek from the Joint Laboratory of Solid State Chemistry of Czech Acad Sci and the University Pardubice for FT-IR measurements. This work was supported by grants 203r94r0024 and 203r96r0876 of the Czech Grant Agency.
References w1x C.J. Balhausen and J.P. Dahl, Acta Chem. Scand. 15 Ž1961. 1333. w2x J.A. Hunter, W.E. Lindsell, K.J. Mc Coullough, R.A. Parr and M.L. Scholes, J. Chem. Soc. Dalton Trans. Ž1990. 2145. w3x M. Horak ´ and D. Papousek, ˇ Infrared spectra and molecular structure. ŽAcademia Praha, 1976. p. 694 Žin Czech.. w4x E.R. Lippincott and R.D. Nelson, Spectrochim. Acta 10 Ž1958. 307. w5x E. Samuel, R. Ferner and M. Birgogne, Inorg. Chem. 12 Ž1973. 881. w6x E. Maslowsky and K. Nakamoto, Appl. Spectrosc. 25 Ž1971. 187. w7x P.M. Druce, B.M. Kingston, M.F. Lappert, T.R. Spalding and R.C. Srivastava, J. Chem. Soc. A Ž1969. 2106. w8x P.M. Druce, B.M. Kingston, M.F. Lappert, R.C. Srivastava, M.J. Frazer and W.E. Newtin, J. Chem. Soc. Ž1969. A2814. w9x M. Spoliti, L. Benciveni, A. Farina, B. Martini and S.N. Cesaro., J. Mol. Struct. 65 Ž1980. 105. w10x M. Moran, ´ Transition Met. Chem. 6 Ž1981. 42. w11x R.J.H. Clark and C.S. Williams, Inorg. Chem. 4 Ž1965. 350. w12x R.J.H. Clark, J. Chem. Soc. 5699 Ž1965.. w13x F.I. Bahayat and W.R. McWhinnie, Spectrochim. Acta A 28 Ž1972. 743. w14x R.J.H. Clark and P.D. Mitchell, J. Chem. Soc. Faraday Trans. 2 71 Ž1975. 515.
Short Communication
Vibrational spectra of niobocene dichloride ˇ J. Holubova´ ) , Z. Cernosek, ˇ I. Pavlık ´ Department of General and Inorganic Chemistry, Faculty of Chemical Technology, UniÕersity Pardubice, nam.Legiı ´ ´ 565, CZ-532 10 Pardubice, Czech Republic Received 29 August 1996; accepted 16 October 1996
Abstract The infrared and Raman spectra of niobocene dichloride were recorded and briefly discussed. The results were compared with the infrared spectra of niobocene di-iodide. The assignment of both infrared and Raman bands was proposed. Keywords: Raman spectroscopy; Niobocene dichloride; Niobocene di-iodide
1. Introduction Niobocene dichloride ŽŽh 5-C 5 H 5 . 2 NbCl 2 . belongs to the group of so called bent metallocenes. The structure of this compound was determined by means of X-ray diffractometry and can be described as C 2v point group symmetry w1x. The object of this article is to study the IR and Raman spectra of niobocene dichloride especially in the far infrared region Ž500–100 cmy1 .. In this low frequency region the spectral bands belong to niobium–cyclopentadienyl ring Žhereafter Nb–Cp. and niobium–chlorine Žhereafter Nb–Cl. bond vibrations. Assuming the cyclopentadienyl rings are rigid disks the compounds approximately have C 2v symmetry and the expected skeletal vibrations are divided as follows: 5A 1 q 3A 2 q 3B1 q 4B 2 . In C 2v symmetry A 1 , B 1 and B 2 type of vibrations are both IR and Raman active, while the A 2 modes are only Raman active. The interpretation of the far IR spectrum is complicated because Nb–Cp and Nb–Cl )
Corresponding author.
stretching vibrations occur in the same spectral region. In order to distinguish between these two types of vibrations we studied the IR spectrum of niobocene di-iodide. The vibrational spectra of title compound in the region 3200–500 cmy1 will be also briefly discussed. In this spectral region we expect intra-ring vibrational modes associated with motion within the C 5 H 5 rings. In local symmetry C 5v the expected fundamental vibrations are 3A 1 q A 2 q 4E 1 q 6E 2 . The A 1 and E 1 vibrational modes are IR and Raman active whereas the E 2 vibrations are only Raman active. 2. Experimental The niobocene dichloride used in our study was a commercial compound ŽFluka. of guarantied purity better than 97%. The same compound was employed for preparation and purification of niobocene diiodide by the method of Hunter and co-workers w2x. The purity of niobocene di-iodide was examined by means of EPR spectroscopy.
0924-2031r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 9 2 4 - 2 0 3 1 Ž 9 6 . 0 0 0 6 2 - 8
148
J. HoluboÕa´ et al.r Vibrational Spectroscopy 14 (1997) 147–150
Transmissional IR spectra were recorded at 1 cmy1 resolution on Perkin-Elmer 684 as nujol mulls between KBr plates with a KBr reference in the region 3200–450 cmy1 and in polyethylene disk in the region 450–200 cmy1 . Kubelka–Munk reflection spectra were measured at 2 cmy1 on FT-IR spectrometer Bio-Rad FTS 45 using KBr in mid-infrared and polyethylene in far-infrared region Ž250 scans were averaged.. Raman spectra of microcrystalline samples were recorded in back scattering geometry on FT spectrometer Bruker IFS-55 with FRA 106 FT Raman equipment using a diode pumped Nd:YAG laser Ž1064 nm. and a nitrogen cooled Ge detector. Microcrystalline powder was placed in a cavity of an aluminium target. The power of the incident light was 25 mWrmm2 , the resolution was 2 cmy1 ; 250 scans were averaged.
Table 1 Proposed assignment for the vibrational spectra of niobocene dichloride and niobocene di-iodide in the intra-ring vibrations range Cp 2 NbCl 2 Ž n Žcmy1 ..
Cp 2 NbI 2 Ž n Žcmy1 ..
Assignment
infrared
Local symmetry ŽC 5v .
Raman
infrared
3120m 3092sh 1449m 1371w — 1125vs 1080m 1067m 855vw 834w 602vw
3097m 3093sh 1440m 1380m 1180vw 1126w 1077vw 1012m 853w 820vs 602vw
3097m — 1447m 1375m 1180vw 1142w 1080vw 1015m 860w 817vs 626vw
A1 E1 E1 E2 E2 A1 E2 E1 E1 A1 E2
nsŽCH. naŽCH. naŽCC. naŽCC. d ŽCH. nsŽCC. g ŽCH. d ŽCH. g ŽCH. g ŽCH. g ŽCC.
n : stretching, d : symmetric out-of-plane deformation, g : asymmetric out-of-plane deformation.
3. Results and discussion 3.1. Infrared spectrum The characteristic bands of fundamental vibrations of cyclopentadienyl rings appear in region
Fig. 1. Infrared spectra of ŽA. niobocene dichloride and ŽB. niobocene di-iodide in the range of intra-ring modes. Shaded area in spectrum ŽA. indicates skeletal niobium-ligands vibrations.
4000–500 cmy1 w3x, Fig. 1. The similarities of the spectra to those of sandwich compounds Cp 2 Fe a Cp 2 Ru w4x and to spectra of related bent metallocenes Cp 2 MX 2 ŽM s Ti, Zr, Hf, V and X s Cl, Br, I. of which the vibrational analyses are available in literature w5–9x allow a simple assignment. In the IR spectrum of niobocene dichloride appear seven active fundamental vibrations which belong to symmetry A 1 and E 1 , Table 1. In spectrum, Fig. 1, we unambiguously identified absorption bands at 3097, 1440, 1126, 1012 and 820 cmy1 . The presence of these bands confirms h 5-coordination of the Cp ring. We deduced from the 1126 cmy1 band that the Cp ring is not substituted w10x. In the spectrum of this compound four very weak bands ŽE 2 . were found, that are forbidden under C 5v symmetry. In the discussed spectral region the spectrum of niobocene di-iodide is identical with the above mentioned spectrum. As expected the change of halogen ligands does significantly not affect the energy of Cp ring vibrations. This result agrees with similar studies of vanadocene dihalides w10x. Table 1 shows the proposed assignment of bands. Energies of skeletal Nb–halogen and Nb–Cp vibrations appear in the range of 500–100 cmy1 . Twelve skeletal modes Ž5A 1 q 3B1 q 4B 2 . are allowed in IR spectra. Comparing the spectra of both
J. HoluboÕa´ et al.r Vibrational Spectroscopy 14 (1997) 147–150
Fig. 2. FT-infrared spectrum of niobocene dichloride.
compounds ŽFigs. 2 and 3. resolves two groups of absorption bands: Ž1. Several bands at 400–375 cmy1 and one band at 303 cmy1 ; exchange of halogen ligands does not effect their energies. Ž2. Several bands at 300–260 cmy1 ; chlorine to iodine ligand exchange shifts the band energies toward low values. On the basis of these results we assume the first group of absorption bands is assigned to stretching
149
vibrations of Nb–Cp and the second one to stretching vibrations of Nb–halogen. On the basis of previously obtained data for some other metallocene dihalides w7,9,10x middle intense bands at 389 and 303 cmy1 in the niobocene dichloride spectrum can be assigned to naŽNb–Cp. of B 2 symmetry and nsŽNb–Cp. of A 1 symmetry. The same conclusion can be drawn for bands of niobocene di-iodide at 392 and 297 cmy1 . Middle intense bands of niobocene dichloride at 377 and 353 cmy1 , the position that does not change with exchange of halogen ligands, could be classified as ring-tilt vibration t ŽCp. with agreement to literature w10x. Strong bands at 289 and 266 cmy1 are related to naŽNb–Cl. of B 1 symmetry and nsŽNb–Cl. of A 1 symmetry and very strong bands in spectrum of niobocene di-iodide at 143 and 125 cmy1 to corresponding vibration of Nb–I bond. This assignment agrees with the observation that the ratio n ŽM-y I.rn ŽM-yCl. ; 0.62 for many tetra- and hexacoordinated complexes w10–14x. The middle intense band at 176 cmy1 in the niobocene dichloride spectrum Ž169 cmy1 in the niobocene di-iodide spectrum. corresponds to a deformation vibration Cp–Nb–Cp in agreement to analyses of IR spectra of vanadocene dihalides w10x.
Table 2 Proposed assignment for the vibrational spectra of niobocene dichloride and niobocene di-iodide in the range of vibrations of metal-ligand bonds
Fig. 3. FT-infrared spectrum of niobocene di-iodide.
Cp 2 NbCl 2 Ž n Žcmy1 ..
Cp 2 NbI 2 Ž n Žcmy1 ..
Raman
infrared
infrared
390m 380sh — 314sh 308vs 288m 270sh 226w 190sh 179m 149m — —
389m 377m 353m — 303s 289vs 266vs 228w 188m 176m 150m — —
392m 380m 353m — 297m — — 206vw — 169m — 143vs 125vs
Local symmetry ŽC 2v .
Assignment
B2 B1 B2
naŽNbyCp. t ŽCp. t ŽCp.
A1 B1 A1 A1 B1 B2 A1 B1 A1
nsŽNbyCp. naŽNbyCl. nsŽNbyCl. t ŽCp. daŽCl–Nb–Cl. daŽCp–Nb–Cp. dsŽCl–Nb–Cl. naŽNb-I. nsŽNb-I.
n : stretching, d : bending, t: tilting.
J. HoluboÕa´ et al.r Vibrational Spectroscopy 14 (1997) 147–150
150
bands at 390 cmy1 and 380 cmy1 can be classified as stretching Nb–Cp bond vibrations as well as a very strong band at 308 cmy1 . The latter band was identified as vibration nsŽNb–Cp. of A 1 symmetry in agreement to several studies w5,6x. In the case of metal–halogen vibrations the authors w7,8x present vibrations of bent metallocenes in Raman spectra which are less intensive as in IR spectra. This conclusion agrees with our observation. The proposed assignment of Raman active skeletal vibrations is given in Table 2.
Fig. 4. FT-Raman spectrum of niobocene dichloride.
The proposed assignment of IR active bands is shown in Table 2. 3.2. Raman spectrum Fig. 4 displays the Raman spectrum of niobocene dichloride, 10 intra-ring vibrations were identified in the spectral region of 3200–500 cmy1 described in Table 1. Detail of Raman spectrum in the spectral region 500–100 cmy1 and its computer deconvolution is presented in Fig. 5. In our opinion middle intense
Fig. 5. Detail of FT-Raman spectrum of niobocene dichloride and its computer deconvolution.
Acknowledgements We would like to thank Dr. V. Smreka ` and Dr. A. Vidourek from the Joint Laboratory of Solid State Chemistry of Czech Acad Sci and the University Pardubice for FT-IR measurements. This work was supported by grants 203r94r0024 and 203r96r0876 of the Czech Grant Agency.
References w1x C.J. Balhausen and J.P. Dahl, Acta Chem. Scand. 15 Ž1961. 1333. w2x J.A. Hunter, W.E. Lindsell, K.J. Mc Coullough, R.A. Parr and M.L. Scholes, J. Chem. Soc. Dalton Trans. Ž1990. 2145. w3x M. Horak ´ and D. Papousek, ˇ Infrared spectra and molecular structure. ŽAcademia Praha, 1976. p. 694 Žin Czech.. w4x E.R. Lippincott and R.D. Nelson, Spectrochim. Acta 10 Ž1958. 307. w5x E. Samuel, R. Ferner and M. Birgogne, Inorg. Chem. 12 Ž1973. 881. w6x E. Maslowsky and K. Nakamoto, Appl. Spectrosc. 25 Ž1971. 187. w7x P.M. Druce, B.M. Kingston, M.F. Lappert, T.R. Spalding and R.C. Srivastava, J. Chem. Soc. A Ž1969. 2106. w8x P.M. Druce, B.M. Kingston, M.F. Lappert, R.C. Srivastava, M.J. Frazer and W.E. Newtin, J. Chem. Soc. Ž1969. A2814. w9x M. Spoliti, L. Benciveni, A. Farina, B. Martini and S.N. Cesaro., J. Mol. Struct. 65 Ž1980. 105. w10x M. Moran, ´ Transition Met. Chem. 6 Ž1981. 42. w11x R.J.H. Clark and C.S. Williams, Inorg. Chem. 4 Ž1965. 350. w12x R.J.H. Clark, J. Chem. Soc. 5699 Ž1965.. w13x F.I. Bahayat and W.R. McWhinnie, Spectrochim. Acta A 28 Ž1972. 743. w14x R.J.H. Clark and P.D. Mitchell, J. Chem. Soc. Faraday Trans. 2 71 Ž1975. 515.