Molecular structures of molybdenum tetra- and pentafluorides

Journal of Molecular Structure 567±568 (2001) 203±210

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Molecular structures of molybdenum tetra- and penta¯uorides q G.V. Girichev a,*, N.I. Giricheva b, O.G. Krasnova a a

Department of Physics, State University of Chemistry and Technology 153000, Ivanovo, Russian Federation b Department of Physical Chemistry, Ivanovo State University, 153025 Ivanovo, Russian Federation Received 14 June 2000; accepted 18 July 2000

Abstract The molecular structures of the MoF4 and MoF5 molecules have been investigated by synchronous gas phase electron diffraction/mass spectrometry. The equilibrium structure of MoF4 is tetrahedral. The molecule MoF5 has C2v symmetry due to the Jahn±Teller effect. Intrinsic periodicity of the M±X bond distance in the MX4 halides along the series of d-elements is discussed. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Molybdenum ¯uorides; Molecular structure; Jahn±Teller effect

1. Introduction Information in the literature concerning the molecular structure of valence-unsaturated compounds is incomplete and at times con¯icting. It is very dif®cult to predict precisely the structure of such molecules, so it seems important to collect experimental information about their structures. Seven molecules of VI A group metal tetra- and pentahalides previously have been investigated by gas-phase electron diffraction (GED). The molecular structures of MoCl4 and MoBr4 were assumed to be Ê (Mo± tetrahedral [1], and bond distances of 2.32(2) A Ê Cl) and 2.39(2) A (Mo±Br) were obtained. In studies of MoBr4 [2] and WCl4 [3], the authors suggested structures that are far from tetrahedral but having C2v symmetry. The molecular structure of CrF4 has been determined to be tetrahedral by both GED [4] and spectroscopic [5] studies. It is important to note q

Dedicated to Professor Marit Trñtteberg on the occasion of her 70th birthday. * Corresponding author.

that vaporisation of these types of halides may lead to a mixture of several molecular species in the gas phase (see Ref. [6] for example), and this possibility has to be taken into account in the re®nement of the experimental data. In a GED study of CrF5 [7], the authors found a structure consistent with C2v symmetry. MoF5, on the other hand, was reported to have D3h symmetry [8]. There is some controversy concerning the structure of MoCl5. According to one study [9] the equilibrium con®guration MoCl5 has C4v symmetry, while a combined GED and ab initio study has concluded that the molecule has C2v symmetry [10]. The available data on the molecular structure of WCl5 is also contradictory. Ezhov and Sarvin [11] described WCl5 as a non-rigid molecule with minimum energy for the C4v symmetry con®guration and with a low pseudo-rotation D3h barrier based on the Berry mechanism. The same authors later reanalysed the same data and concluded with a C4v structure for WCl5 [12]. A more recent GED study indicated that the WCl5 molecule has D3h symmetry [13]. The authors suggested that the differences between the

0022-2860/01/$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0022-286 0(01)00553-1

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WCl5 and the CrF5 and MoCl5 structures are due to strong spin-orbit couplings that quench the distortion of high molecular symmetry. It may be that the differences between the conclusions concerning the symmetry of the tetra- or pentahalides of the group VI A metals mentioned above could be caused the uncertainty about the vapour composition. The present work is devoted to an investigation of molybdenum tetra¯uoride and penta¯uoride by means of gas-phase electron diffraction accompanied by simultaneous mass spectrometric measurements (GED/MS). The preliminarily results have been published earlier [14,15]. 2. Experimental Solid MoF3 and MoF5 samples were used for the investigation of the MoF4 and the MoF5 molecular structures. Preliminarily MS measurements showed that the vapour over solid MoF3 consisted of MoF4 and MoF5, and signi®cant amounts of molybdenum oxy¯uorides. The oxy¯uorides could be removed by heating the sample in vacuum for about 1±2 h at a temperature of 30±508 less than the temperature of the GED experiment. The saturated vapour of MoF5 consisted of trimeric molecules. The monomers could, however, be obtained by overheating the vapour at a temperature of about 500 K. The composition of the vaporised samples of MoF3 and MoF5 could make it dif®cult to get clean diffraction pattern recordings if conventional GED is used. Therefore, the combined GED/MS method has been applied to avoid the uncertainty about the vapour composition [16,17].. Owing to high hygroscopicity of MoF4 and MoF5 a dry chamber was used to ®ll the effusion cells with samples. Electron diffraction patterns were obtained at a nominal accelerating voltage of 75 kV and a beam current of 2 mA for nozzle-to-plate distances of L1 ˆ 598 and L2 ˆ 338 mm. The process of eliminating the oxyhalides preceded the recording of the diffraction patterns. A molybdenum double effusion cell was used for the vaporisation of pure MoF5. The temperature of the ®rst chamber, with the MoF5 sample, was 333(5) K.

The temperature of the second chamber was 551(10) K. The relative intensities of the ion peaks in the mass spectra, measured during plate recordings, were 14, 17, 26, 29, 100, and 26 for Mo 1, MoF 1, MoF21, MoF31, MoF41 and MoF51, respectively. MoF4/MoF5 (from MoF3) was vaporised from a nickel effusion cell at 943(5) K. A mass spectrum was measured simultaneously with the plate recordings (relative intensities were 18, 24, 26, 100, 57 and 5 for the ion peaks Mo 1, MoF 1, MoF21, MoF31, MoF41 and MoF51, respectively). This spectrum corresponded to a vapour composition of 82 mol% MoF4 and 18 mol% MoF5.

3. Structural analysis Least-square structure re®nements were carried out with the modi®ed KCED35 program, using diagonal weight matrices. 3.1. Molybdenum penta¯uoride According to mass spectra obtained during plate recordings, the investigated vapour consisted of MoF5 molecules. In the LS analysis models with D3h, C4v and C2v symmetry were considered. The internal coordinates used in the vibrational analysis are shown in Fig. 1. The exact vibrational frequencies of MoF5 are unknown. The analysis of a combination of IR spectra of matrix isolated MoF5, Raman and IR spectra of solid MoF5 [18], as well IR spectra of MoF5 obtained during photolysis of MoF6 in Ar matrix [19] allowed estimation of the region of stretching frequencies (650±770 cm 21) and the region of bending frequencies (110±260 cm 21). The estimated force ®eld that corresponded to these regions of frequencies was used

Fig. 1. The considered models of the MoF5 molecule and the internal coordinates used in the vibrational analysis.

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205

Table 1 The results for the molecular structure of MoF5 from LS analysis using different models (paranthesized values are s LS) D3h Ê) ra (A Mo±F1 Mo±F2 Mo±F4 F1 ±F2 F1 ±F4 F2 ±F3 F4 ±F5 F2 ±F5 /F2 ±Mo±F3 (8) /F4 ±Mo±F5 (8) /F1 ±o±F2 (8) Rf (%) a

1.845(4) 1.821(3) ± 2.585(1) 3.128(5) 3.679(9) ± ± ± ± ± 4.73

Ê) 1 (A 0.068(1) 0.046 a 0.067(1) 0.045 ± 0.167(3) 0.132 0.184(8) 0.182 0.076(11) 0.061 ± ±

C4v Ê) ra (A

Ê) 1 (A

1.735(3) 1.851(5) ± 2.786(13) ± 3.595(9) ± 2.521(6) 155.1(1.3) 87.3(0.3) 102.4(0.6) 4.93

0.049(2) 0.044 0.051(2) 0.046 ± 0.259(17) 0.139 ± 0.393(68) 0.082 ± 0.149(5) 0.207

C2v Ê) ra (A 1.732(2) 1.863(2) 1.840(2) 2.659(6) 3.049(7) 3.687(5) 3.201(13) 2.541(3) 168.1(0.6) 122.6(0.8) ± 2.51

Ê) 1 (A 0.0467(1) 0.045 0.049(1) 0.047 0.048(1) 0.045 0.158(6) 0.156 0.176(8) 0.172 0.079(8) 0.075 0.172(12) 0.181 0.154(2) 0.151

Calculated from force ®eld.

for the calculation of mean square amplitudes of vibration, 1, and shrinkage corrections. The results of the LS analysis are given in Table 1. The model with C2v symmetry seems to be more preferable because of a low goodness-of-®t factor, Rf. The good agreement between the experimental and calculated amplitudes for the C2v model and the disagreement between these values for the other models, also suggests that the C2v models is the best. The experimental and theoretical intensities, as well as the corresponding radial distribution curves, are in

a good agreement with each other (see Figs. 2 and 3, respectively). The obtained molecular structure of MoF5 is similar to the structure of CrF5 and MoCl5. These structures were so unusual that the author [20] was in doubt about vapour composition of the Cr and Mo pentahalides. The IR spectrum of matrix isolated CrF5 has been studied by Hope et al. [21]. They concluded that the vapour contained CrF4 and CrF6 species instead of CrF5. The set of ions in the mass spectrum of MoF5 did not contradict this composition. Taking

Fig. 2. Experimental (dots) and theoretical (solid) molecular intensity curves and the difference (exp. 2 theor.) for MoF5 (C2v) at two nozzle-toplate distances, 598 and 338 mm.

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Fig. 3. Experimental (dots) and theoretical (solid) radial distribution curves and the difference (exp. 2 theor.) for MoF5 (C2v).

into account this circumstance, we carried out LS analysis using a theoretical intensity of sM…s†theor ˆ asM…s†MoF4 1 …1 2 a† sM…s†MoF6 s: Two types of re®nements have been made. In the ®rst, the parameters of both the MoF4 and the MoF6 molecules and a were re®ned independently. In the second type of re®nement, the parameters of MoF6 were calculated for the temperature of the ED experiment using published data [22], and these parameters were kept ®xed. The Rf values obtained (4.5 and 6.8% for the ®rst and for the second type of re®nement, respectively) and the geometrical disagreement between the structural parameters for both the MoF4 and the MoF6 molecules, as well as the unrealistic vapour composition we obtained, 89(9) mol% MoF4, indicated that this idea concerning the vapour composition of MoF5 probably not is correct. 3.2. Molybdenum tetra¯uoride The vapour studied consisted of MoF4 and MoF5 according to the mass spectrum that was measured simultaneously with the plate recordings. Taking into account this circumstance, the theoretical intensity function was expressed as a sum of the partial contributions from each of the molecular species that had been discovered in the vapour. sM…s†theor ˆ a sM…s†MoF4 1 …1 2 a† sM…s†MoF5 :

The contribution of MoF5 to the intensity curve was calculated using internuclear distances and amplitudes obtained in the separate experiment described above. Preliminarily, the parameters were scaled from 551 to 943 K and they were ®xed in LS re®nements. Models of MoF4 with molecular structures having Td, D2d, D4h, C4v, C2v and C3v symmetry were considered (Fig. 4). The LS analysis showed that the vapour contains both MoF4 and MoF5 species. The amount of MoF5 re®ned to a value around 15 mol%, depending on the assumed model of MoF4. Fig. 5 shows the radial distribution curve for a model containing MoF4 and MoF5 species. If the Ê is assumed to correspond to the shoulder at 2.6 A F±F non-bonded distances in a D4h or a C4v model Ê has to be assigned of MoF4, the strong peak at 3.1 A to the F±F distance with multiplicity of 2 in MoF5. In Ê seems to be this case, the area under the peak at 3.1 A unrealistically large, and this fact allows us to reject the D4h and the C4v models. Next, the vibrational analysis was carried out for models with symmetry Td, D2d, C2v and C3v. The experimental values of the vibrational frequencies for MoF4 are not known; therefore, we estimated these values using the relationship between force constants in molecules with the formula MX4 [23] and the experimental frequencies for ZrF4 [24,25]. For models with D2d, C2v and C3v symmetry, the set of frequencies was changed according to the vibrational representation. The estimated frequencies were used to calculate amplitudes of vibration and shrinkage-corrections. The results of the LS re®nements are shown in Table 2. The Rf values alone do not make it possible to pick a preferred model. However, the C3v model can be rejected because of the re®ned values of the amplitudes corresponding to the stretching vibrational frequencies that should be above 800 cm 21. These values are signi®cantly higher than the corresponding values in CrF4, and these results contradict the tendency of stretching frequencies decreasing in a subgroup if the metal of the ®rst transition series is substituted with a second series metal. The C2v model could be excluded as well because within the largest uncertainty limits the re®ned parameters are in agreement with both Td and D2d models. Comparison of the results for Td and D2d models suggest a better ®t for the Td model, since the lowering of the molecular

G.V. Girichev et al. / Journal of Molecular Structure 567±568 (2001) 203±210

207

Fig. 4. The models considered for the MoF4 molecule.

symmetry strongly increases the s LS for the r(F±F) Ê ) for the distance. The calculated shrinkage (0.015 A Td model is in agreement with the experimental value Ê , and this is an additional argument in 0.017(10) A favour of the Td model. The experimental and theoretical intensity curves for the Td model are shown in Fig. 6. The GED data for the MoF4 (Td) molecule were used for the determination of its vibrational frequencies by the method described in [26]. The full set of equations for the tetrahedral molecules XY4 is given in

Fig. 5. Experimental (dots) and theoretical (solid) radial distribution curves and the difference (exp. 2 theor.) for MoF4 (Td).

another publication [27]. Two approaches to the calculation of the force ®eld of MoF4 were used Ð by Larnaudie [28] and by using a modi®ed valence force ®eld [29]. The results are listed in Table 3, where also the results of MP2 ab initio calculation [30] are listed.

4. Discussion Both MoF4 and MoF5 molecules have a nonspherical d-shell in the central atom, and this feature could affect the arrangements of the ¯uorine atoms. The Td con®guration of MoF4 and D3h con®guration of MoF5 corresponds to minimal repulsions between ligands, but the ribronic interaction can lead to a distortion of the high symmetry of the molecules. Our GED work shows that the MoF4 molecule belongs to the Td symmetry despite a partially ®lled 4d-subshell for molybdenum. A simple explanation for the tetrahedral structure of MoF4 could be given by employing the crystal ®eld theory. In a tetrahedral ligand ®eld two electrons will occupy d z2 and d2x 2 y2 orbitals of the Mo 41 ion. One may conclude that the electron density of these orbitals are likely to be distributed over the shell of the central atom, and so has the symmetry Oh.

c

b

a

4 ± 6 ± ±

0.0526(14) 0.053 c 4 ± ± 0.2205(111) 0.217 2 ± 4 ± ±

1.8491(9) b ± 2.9895(15) ± ± ± ± ± 5.68 20.0(3.6)

D2d k

Ê) 1 (A

Ê) ra (A

Multiplicity. Parenthesized values are s LS. Calculated from force ®eld.

Mo±F1 Mo±F2 F1 ±F2 F2 ±F3 F3 ±F4 a (8) b (8) g (8) Rf (%) MoF5 (%)

Td ka 1.8481(8) ± 2.7722(178) 3.0941(73) ± 98.4(0.8) 115.2(0.4) ± 5.52 14.9(3.6)

Ê) ra (A 0.0535(13) 0.054 ± 0.1873(114) 0.207 0.1665(114) 0.186 ±

Ê) 1 (A

Table 2 Results for the molecular structure of MoF4 from LS analysis for different models

1 3 3 3 ±

C3v k 1.8919(176) 1.8335(44) 2.7994(225) 3.1114(93) ± 98.8(1.1) 117.7(0.6) ± 5.47 10.2(4.9)

Ê) ra (A 0.0451(58) 0.052 0.0493(58) 0.056 0.2122(117) 0.224 0.1479(117) 0.160 ±

Ê) 1 (A 2 2 1 4 1

C2v k

1.8530(536) 1.8439(560) 2.8258(5281) 3.0850(207) 2.7500(5190) 97.8(26.0) 114.7(1.2) 101.0(24.2) 5.68 17.0(5.9)

Ê) ra (A

0.0530(43) 0.053 0.0560(73) 0.056 0.2278(1848) 0.237 0.1718(1848) 0.180 0.1971(1848) 0.206

Ê) 1 (A

208 G.V. Girichev et al. / Journal of Molecular Structure 567±568 (2001) 203±210

G.V. Girichev et al. / Journal of Molecular Structure 567±568 (2001) 203±210

Fig. 6. Experimental (dots) and theoretical (solid) molecular intensity curves and the difference (exp. 2 theor.) for MoF4 (Td) at two

Thus this density does not perturb the tetrahedral arrangement of the ligands of MoF4. Experimental data for MoF5 may indicate the presence of a Jahn±Teller effect. From the point of view of crystal ®eld theory, the d-electron of Mo 51 may occupy either dxz or dyz ±AO (assuming axial F atoms lie along the z-axis) that must be equal in energy in the D3h symmetry ®eld. When the symmetry of the molecule becomes lower, the orbitals are no longer degenerate. According to our GED study, the equilibrium structure of MoF5 is characterised by C2v symmetry, and the overall picture of distortion (see Fig. 1c) is in agreement with the dyz ±AO occupied by an electron. This results in Mo±F4 and Mo±F5 equatorial bond elongation as compared to the Mo± F1 bond. The presence of two chemically nonequivalent Mo±F bonds appears to be a peculiar feature of the experimentally determined structure of MoF5. The Ê ) is signi®cantly shorter than Mo±F1 distance (1.819 A

209

all the other bonds in both MoF5 and MoF6. Thus, the geometrical con®guration of MoF5 is similar to that of CrF5 and MoCl5. The M±F1 bond is the shortest and the M±F4 bond is the longest, the valence angle F4 ±M±F5 is larger than 1208 and the F2 ±M±F3 angle is less than 1808. It is interesting to analyse the bond distance variation in the transition metals series tetrahalides. The variation in the r(M±X) bond distance of the diand trihalides of the ®rst transition series metals are known [31,32]. In the di¯uorides, the crystal ®eld stabilisation energies are zero for CaF2 (d 0), MnF2 (d 5), and ZnF2 (d 10); in these molecules, the r(M±F) distances are longer than in the other di¯uoride molecules. In the tri¯uorides of the ®rst transition metal series, the crystal ®eld stabilisation energies are zero for ScF3, (Sc 31, d 0), FeF3 (Fe 31, d 5) and GaF3 (Ga 31, d 10). A smooth curve re¯ecting the Ê ) versus change of the ionic radii (r(m 31) 1 1.42 A atomic number passes through these three ions. The values of the r(M±F) distance for the other molecules of this row lie under this curve [32]. Not much information on the gas-phase structures of transition metal tetra¯uorides are known. Nevertheless, the data available are well described if crystal ®eld theory is applied. Fig. 7 demonstrates the behaviour of the r(M±X) distance in the series of ZrX4, NbX4, and MoX4 (X ˆ F, Cl). The ZrX4 (Xr 41, d 0) molecule with the longest r(M±X) distance opens a series exhibiting a double periodicity. In going through NbX4 (Nb 41, d 1) to MoX4 (Mo 41, d 2), the r(M±X) distance decreases, because the low-lying dz2 and dx2±y2 orbitals are being occupied. From MoX4 to RhX4 (Rh 41, d 5), we should anticipate an increase in the bond distance and, after that, a decrease again. Thus the double periodicity in the

Table 3 Parameter values for MoF4 obtained from GED data together with values from ab initio molecular orbital calculations

Ê) reh (Mo±F) (A V1 (cm 21) V2 (cm 21) V3 (cm 21) V4 (cm 21)

GED aprroach [28]

GED approach [28]

Ab initio MP2 [30]

1.828(3) 697(70) 128(15) 736(22) 164(21)

1.827(3) 689(73) 122(15) 726(40) 171(33)

1.861 706 a 136 698 109

a Ê , 1(F±F) ˆ 0.267 A Ê , and d (F±F) ˆ 0.0414 A Ê . The two last values are surprisThe set of ab initio frequencies leads to 1(Mo±F) ˆ 0.056 A ingly exceeded by the experimental ones. It seems that the bending frequency v4 calculated in Ref. [30] is too low.

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Fig. 7. The shortening of the bond distance in the MX4 halides along the 2 series of d-elements.

occupation of the central ion d±AO's manifests itself as the deviation from the monotonic shortening of the bond distance in the MX4 halides along the series of d-elements. References [1] V.P. Spiridonov, G.V. Romanov, Vestn. MGU (Russian) 1 (1969) 65. [2] Yu.S. Ezhov, S.A. Komarov, Zh. Strukt. Khim. (Russian) 34 (3) (1993) 47. [3] Yu.S. Ezhov, S.A. Komarov, Zh. Strukt. Khim. (Russian) 25 (1) (1984) 82. [4] L. Hedberg, K. Hedberg, G.L. Gard, J.O. Udeaja, Acta Chem. Scand. A 42 (1988) 318. [5] J. Jacobs, H.S.P. MuÈller, H. Willner, E. Jacob, H. BuÈrger, Inorg. Chem. 31 (1992) 5357. [6] R.V. Siepmann Von, H.-G. Schnering, H. Shafer, Angew. Chem. 14 (1967) 650. [7] E.J. Jacob, L. Hedberg, K. Hedberg, H. Davis, G.L. Gard, J. Phys. Chem. 88 (10) (1984) 1935. [8] V.P. Spiridonov, G.V. Romanov, Vestn MGU (Russian) 1 (1967) 98. [9] Ischenko, A.A., Ivashkevich, L.S., Spiridonov, V.P., Demidov, A.V., Ivanov, A.A, Chemistry and technology of molybdenum and tungsten (IV Meeting), Tashkent, 1980, 50p. [10] K. Faegri, K.-G. Martinsen, T.G. Strand, H.V. Volden, Acta Chem. Scand. 47 (1993) 547.

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