(M = Cr, Mo, W, and Element 106, Sg): The Electronic Structure and

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J. Phys. Chem. 1996, 100, 8748-8751

Group 6 Dioxydichlorides MO2Cl2 (M ) Cr, Mo, W, and Element 106, Sg): The Electronic Structure and Thermochemical Stability V. Pershina* and B. Fricke Theoretical Physics Department, UniVersity of Kassel, D-34132 Kassel, Germany ReceiVed: August 29, 1995; In Final Form: December 1, 1995X

Results of the Dirac-Slater discrete-variational calculations are presented for the group 6 dioxydichlorides, MO2Cl2 (M ) Cr, Mo, W, and element 106, Sg). Estimates of the bond strengths and stability toward thermal decomposition have shown single molecules of O2Cl2 to be as stable as those of MoO2Cl2. Calculated energies of electronic charge-transfer transitions proved to be lower than those for WO2Cl2. The stability of the +6 oxidation state is shown to decrease in going from the W to Sg compound.

I. Introduction Halides and oxyhalides of element 106, Sg, and its analogs are the species whose volatility is to be studied experimentally1 by applying the gas-phase chromatography technique. Since there are species of the lighter analogs of the Sg compounds which are thermally unstable, investigation of the electronic structure and thermochemical stability of the Sg halides and oxyhalides is important. In our previous publications,2,3 we presented results of the Dirac-Slater discrete-variational (DS DV) calculations of the electronic structure of MCl6 and MOCl4, where M ) Mo, W, and Sg, and estimates of the thermochemical M-Cl and MdO bond energies. According to these data, by analogy with MoCl6 and MoOCl4, both SgCl6 and SgOCl4 will be unstable and decompose into SgCl5 and SgOCl3, respectively. Thus, the better choice for the study of gas-phase chromatography is probably the dioxydichloride species, which are known to be more stable for Mo and W toward thermal decomposition than the respective MOCl4. They have also more negative enthalpies of formation (-151.60 and -163.96 kcal/mol for MoO2Cl2 and WO2Cl2, respectively, while ∆Hf for MoOCl4 and WOCl4 are -140 and -152 kcal/mol, respectively). In macroamounts, the stability of MoO2Cl2 and WO2Cl2 is, nevertheless, different. While the former compound is stable, the latter is known to decompose at 260 °C simultaneously with its sublimation according to the equation

TABLE 1: Geometrical Parameters for MO2Cl2 Molecules (M ) Cr, Mo, W, and Sg) in the Gas Phase (C2W Symmetry) molecule

RMdO, Å

RM-Cl, Å

∠OMCl

∠ClMCl

ref

CrO2Cl2 MoO2Cl2

1.581 1.698

2.126 2.259

113.3 112

WO2Cl2

1.710

2.270

SgO2Cl2

1.772

2.330

108.5 104 ( 2 109 ( 3 104 107 ( 2 104 108

22 20 19 21 19 estimate estimate

112 112

Thermodynamic measurements of the dioxydichlorides are numerous and summarized in refs 4 and 5. The electronic structure and the absorption spectra for CrO2Cl2 and MoO2Cl2 have been studied in refs 6 and 7 by applying the SCF XRscattered waves (SCF-XR-SW) method. In this publication, we present results of the relativistic Dirac-Slater discretevariational calculations of the electronic structure of all the group 6 dioxydichlorides and their analysis with respect to the stability of these compounds. In section II some details of the calculations are given. Results and their discussion are presented in section III.

the DS DV method. A description of the method and its application to 104-106 halides and oxyhalides can be found elsewhere.8,9-13,2-3 Both full electron and frozen-core approximations were used. The basis set included the valence ns, np1/2, np3/2, (n - 1)d3/2, and (n - 1)d5/2 orbitals. Calculations of the molecular integrals were done numerically using 9000 integration points. Double point group representations were used for the molecular symmetry orbitals. Mulliken population analysis14 was applied to study the distribution of the electron density and bonding. The overlap population (OP) parameter is used as a measure of covalency. B. Structures of MO2Cl2 (M ) Cr, Mo, and W) and Input Parameters. The species which are formed in the gas-phase chromatography experiments1 are separate molecules produced on the one-atom-at-a-time scale. Gaseous homologs of the Sg dioxydichloride, CrO2Cl2, MoO2Cl2, and WO2Cl2 are known to be volatile compounds having a monomeric structure in the gas phase. Structural information was obtained from a study of infrared spectra of the vapor and monomers in matrices15-19 as well as from electron diffraction measurements.20,21 Results of the measurements are summarized in Table 1. Absorption spectra have been studied in refs 6, 7, and 19. The calculations were performed for the C2V geometries and interatomic distances indicated in Table 1. For MoO2Cl2 and WO2Cl2, both values of ∠OMCl obtained by the different measurements were used. The bond lengths for SgO2Cl2 were estimated taking into account an increase in the covalent radii of Sg relative to its analogs (see ref 23) and relativistic bond contraction: RSg-Cl is 0.06 Å larger than RW-Cl and RSgdO is 0.062 Å larger than RWdO.

II. Method and Details of the Calculations

III. Results of the Calculations and Discussion

A. Dirac-Slater Discrete-Variational Method. Calculations of MOCl4 (M ) Mo, W, and Sg) were performed using

A. MO Energies and Electronic Spectra of MO2Cl2, Where M ) Cr, Mo, W, and Sg. The energy eigenvalues obtained as a result of the calculations for the dioxydichlorides of Mo, W, and Sg are given in Table 2. The calculated MO

2WO2Cl2(g) f WO3(s) + WOCl4(g)

X

(1)

Abstract published in AdVance ACS Abstracts, April 15, 1996.

S0022-3654(95)02530-5 CCC: $12.00

© 1996 American Chemical Society

Group 6 Dioxydichlorides MO2Cl2 (M ) Cr, Mo, W)

J. Phys. Chem., Vol. 100, No. 21, 1996 8749

Figure 1. Energies of the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) in MOCl4 and MO2Cl2, where M ) Mo, W, and Sg.

TABLE 2: Orbital Energies for MoO2Cl2 (M ) Mo, W, and Sg) MoO2Cl2

WO2Cl2

SgO2Cl2

MO

-Eorb, eV

MO

-Eorb, eV

MO

-Eorb, eV

51D5 50D5 49D5 48D5 47D5a 46D5b 45D5 44D5 43D5 42D5 41D5 40D5 39D5 38D5 37D5 36D5 35D5 34D5 33D5

1.89 2.66 3.98 4.43 4.99 8.46 8.52 8.58 8.90 8.99 9.39 10.03 10.88 11.22 11.28 11.40 11.75 21.24 21.36

67D5 66D5 65D5 64D5 63D5a 62D5b 61D5 60D5 59D5 58D5 57D5 56D5 55D5 54D5 53D5 52D5 51D5 50D5 49D5

1.55 2.18 3.18 3.82 4.56 8.35 8.65 8.70 8.98 9.13 9.55 10.44 10.94 11.07 11.19 11.32 11.66 21.51 21.68

83D5 82D5 81D5 80D5 79D5a 78D5b 77D5 76D5 75D5 74D5 73D5 72D5 71D5 70D5 69D5 68D5 67D5 66D5 65D5

1.71 2.54 2.97 3.74 4.79 8.30 8.85 8.89 9.06 9.23 9.81 10.60 10.83 11.07 11.38 11.58 11.84 21.78 22.01

a

LUMO. b HOMO.

energies for MoO2Cl2 agree with those obtained in ref 7 using the nonrelativistic SCF-XR-SW method. The present calculations give, however, a smaller value of the energy gap, ∆E, between the highest occupied (HOMO) and the lowest unoccupied MO (LUMO) in comparison with that of ref 7. The MO composition nevertheless shows an essential difference for the HOMO, which in the case of the DS DVM calculations is a mixture of the 2p(O) (77.4%) and 3p(Cl) (26.5), while the composition of the HOMO obtained in the SCF-XR-SW calculations7 is 96.57% of the 3p(Cl) and only 2.37% of the 2p(O). In going from MoO2Cl2 to SgO2Cl2, the HOMO becomes nearly fully a 2p(O) orbital. For lower lying bonding orbitals, the present calculations give higher contributions of the metal orbitals in bonding than those of Topol et al.7

It is interesting that in contrast to the calculations2,3 of a series of the other group 6 molecules, MCl6 or MOCl4 (M ) Mo, W and Sg), the energies of the virtual MO’s are lowered in SgO2Cl2 in comparison with WO2Cl2. In addition, the values of the energy gap ∆E in MO2Cl2 are much higher than those of the corresponding MOCl4: 3.47, 3.80, and 3.51 eV for MoO2Cl2, WO2Cl2, and SgO2Cl2, respectively, in comparison with 1.01, 1.90, and 2.40 eV for MoOCl4, WOCl4, and SgOCl4, respectively (see Figure 1). This is reflected in the energies of the charge-transfer transitions calculated via the transition state procedure (Table 3). Our calculated energies of the charge-transfer transitions agree better with experiment19 than those obtained by using the SCFXR-SW method,7 where the energies are enhanced due to a larger ∆E. The present calculations give the lowest charge-transfer transitions at 30 684, 33 344, and 32 150 cm-1 for the Mo, W, and Sg compound, respectively. If these values are correlated with the reduction potentials in aqueous solutions, this would mean a decrease in the stability of the +6 oxidation state in going from the W to Sg compound. The character of higher charge-transfer transitions for SgO2Cl2 will differ from those of the lighter analogs due to a larger admixture of metal orbitals into the formed MO (e.g., 21.7% of the metal in the 72D5 MO). B. Charge-Density Distribution and Bonding. Calculated effective atomic charges, overlap populations, and dipole moments are given in Table 4. These data are reminiscent of those for the group 6 MOCl4: the Sg dioxydichlorides has the lowest effective charge and the highest OP, or covalency. The MO2Cl2 compounds are slightly less covalent than the MOCl4 ones (see total OP in Table 4 and Table 1 of ref 3). As in MOCl4, an increase in covalency comes at the expense of the metal-oxygen interaction, while the metal-chlorine bonding decreases in going from the W to Sg compound. So, as it was in the series of the other group 6 compounds, the hexachlorides and oxytetrachlorides, the Sg-Cl bond will be weaker than the W-Cl. Thermochemical bond energies, estimated in the same way as was done in ref 3 for MOCl4, by using the values of the effective charges and OP, are given in Table 5. (∆Hdiss for the lighter homologs of the Sg oxyhalide have been calculated via the Born-Haber cycle). MO2Cl2 have dipole moments, which are higher than those of the corresponding MOCl4, but having the same sign (in ref 7 the signs of the dipole moments of MO2Cl2 and MOCl4 are different), and slightly higher than those obtained in ref 7. The values of the dipole moments increase in going from CrO2Cl2 to SgO2Cl2. C. Thermodynamic and Thermochemical Stability. The fact that the dioxydichlorides of Mo and W are more stable than the corresponding oxytetrachlorides cannot be accounted

TABLE 3: Observed and Calculated Charge-Transfer Bands (cm-1) for Isolated MO2Cl2 Species compd

transition

present calca

calcb

MoO2Cl2

46D5 f 47D5 44D5 f 47D5 41D5 f 47D5 40D5 f 47D5 65D5 f 63D5 60D5 f 63D5 57D5 f 63D5 56D5 f 63D5 78D5 f 79D5 76D5 f 79D5 73D5 f 79D5 72D5 f 79D5

30 684 32 752 37 663 43 067 33 344 36 181 42 198 48 038 32 150 34 917 41 050 47 261

39 760

WO2Cl2

SgO2Cl2

42 744 49 197

expc 32 680 38 610 44 400 35 970 42 730 48 540

comments O, Cl f Mo (lowest) Cl f Mo (center) O, Cl f Mo (center) O, Cl f Mo (center) O, Cl f W (lowest) Cl f W (center) O, Cl f W (center) O, Cl f W (center) O f Sg (lowest) O, Cl f Sg (center) O, Cl f Sg (center) M, O, Cl f Sg (center)

Our calculated data are shown here for ∠OMCL ) 104° for compounds of Mo, W, and Sg. For these compounds with larger angles, indicated in Table 1, the energies will be about 700, 400, and 250 cm-1, respectively, lower. b Reference 7. c Reference 19. a

8750 J. Phys. Chem., Vol. 100, No. 21, 1996

Pershina and Fricke

TABLE 4: Effective Charges (Q), Overlap Populations (OP), and Dipole Moments (µ) for MO2Cl2a parameter

CrO2Cl2

MoO2Cl2

WO2Cl2

SgO2Cl2

QM QO QCl OP(Md2O) OP(M-2Cl) OP(tot) µ, D

0.96 -0.31 -0.17 0.92 0.62 1.61 0.88

1.03 -0.34 -0.18 1.07 0.64 1.72 1.04

1.08 -0.37 -0.19 1.38 0.86 2.23 1.35

0.97 -0.35 -0.13 1.48 0.85 2.34 1.83

a For the compounds of Mo, W, and Sg with ∠OMCl of 109, 107, and 108°, respectively, the values of QM are =0.005 larger and OP are 0.005 smaller. The dipole moments are 0.97, 1.27, and 1.75 D, respectively.

TABLE 5: Energies of the Ionic (Ei) and Covalent (Ec) Contributions to a Single Metal-Ligand Bond Energy and the Dissociation Energies (∆Hdiss) (in eV) in MO2Cl2 (M ) Cr, Mo, W, and Sg) MdO

M-Cl

compd

Ei

Ec

Edissa

Ei

Ec

Edissa

∆Hdiss

CrO2Cl2 MoO2Cl2 WO2Cl2 SgO2Cl2

2.69 2.98 3.34 2.74

2.77 3.88 4.41 4.60

5.46 6.86 7.75 7.45

1.10 1.31 1.30 0.78

1.87 2.32 2.75 2.65

2.97 3.64 4.05 3.43

17.30 21.08 23.52 21.6-23.0

a

Ediss ) ∆H′diss(thermochemical).

TABLE 6: Formation Enthalpies (in kcal/mol) of Different Oxyhalides of Cr, Mo, W, and Sga M

MO2Cl2(g)

MO3(s)

MOCl4(g)

∆Hreaction(1)

Cr Mo W Sg

-128.61 -151.60 -163.96 -121.72b

-140.77 -178.06 -201.48 =-200.00c

-66.66 -140.00 -152.00 -114 ( 6d

-14.86 -25.56 =-70.60e

a Reference 24. b Calculated in the present work. c Taken approximately as the value for WO3. d Reference 3. e Estimated in the present work.

for by a higher covalency of the metal-ligand bonds in the former compounds: the partial OP data for the MdO and M-Cl bonds are similar in MO2Cl2 and MOCl4 (see Table 4 and Table 1 in ref 3). Thus, the M-Cl bond energy in MoO2Cl2 is only 0.13 eV larger than that of MoOCl4, though the latter compound is known to decompose at 25 °C losing Cl and transforming into a compound of the pentavalent Mo, MoOCl3. Obviously, the much higher stability of MoO2Cl2 is explained by a much higher value of ∆E, which is 2.46 eV larger than that of MoOCl4. Taking this into account, SgO2Cl2 should also be more stable than SgOCl4, though their M-Cl bond strengths are similar. Separate molecules of WO2Cl2, produced on the one-atomat-a-time scale, will also be very stable. The decomposition of WO2Cl2 occurs only for macroamounts, because two molecules are needed for reaction 1. The formation enthalpies of the components of reaction 1 are given in Table 6. ∆Hf(SgO2Cl2) has been calculated via the Born-Haber cycle, using the estimated ∆Hdiss(SgO2Cl2) and ∆Hf of the Sg metal from ref 25. The calculated enthalpy of reaction 1 is given in the right column of Table 6. (The enthalpies for reaction 1 given by other authors26-28 are 36.0, 30.5, and 29 kcal/mol.) Thus, the equilibrium of reaction 1 is shifted to the right for WO2Cl2. Provided two molecules of SgO2Cl2 occur at the same time, the decomposition of the compound is even faster. Nevertheless, at the very low production rate of Sg (1 atom approximately every 30 min), this will not happen. Thus, in the proposed gasphase chromatography experiments the dioxydichloride of Sg will be rather stable.

D. Volatility of the Group 6 Dioxydichlorides. The equilibrium vapor pressure has been measured for both MoO2Cl2 and WO2Cl24,5 showing that the volatility decreases with increasing atomic number of the metal atom. Both compounds are less volatile than WOCl4. The lower volatility of the dioxydichlorides compared to the oxytetrachlorides is explained by (i) slightly lower covalency of the former compounds and (ii) much higher values of the dipole moments. The lower volatility of WO2Cl2 compared to MoO2Cl2 is a result of a larger dipole moment in the former compound. Due to a larger dipole moment, an enhanced dipole-dipole intermolecular interaction results in an increase in the total energy of the intermolecular interaction leading finally to a decrease in volatility. Taking this into account, one can expect a lower volatility of SgO2Cl2 in comparison with the lighter analogs due to a larger value of the dipole moment. IV. Conclusion Results of the DS DVM calculations of the gaseous MO2Cl2 (M ) Cr, Mo, W, and Sg) molecules and estimates of the bond dissociation energies have shown that the metal-oxygen and metal-chlorine bonding in these compounds is similar to that of the respective group 6 MOCl4 compounds. The higher stability of the former compounds toward thermal decomposition and transfer to those with a lower oxidation state of the metal atom is accounted for by much larger values of ∆E. Thus, SgO2Cl2 will be rather stable with a volatility being lower than that of both MoO2Cl2 and WO2Cl2 due to the larger value of the dipole moment. The estimated value of the enthalpy of formation of gaseous SgO2Cl2 is -121.72 kcal/mol, which is less negative than ∆Hf for the dioxydichlorides of Mo and W. Acknowledgment. The calculations were performed on the IBM ES1000 621 at the Gesellschaft fu¨r Schwerionenforschung (GSI). V.P. thanks Deutsche Forschungsgemeinschaft (DFG) for financial support and GSI for hospitality. References and Notes (1) (a) Ga¨ggeler, H. W.; Jost, D. T.; Kovacs, J.; Scha¨del, M.; Bru¨chle, W.; Becker, U.; Kratz, J. V.; Eichler, B.; Hu¨bener, S. GSI Scientific Report 1991, GSI 92-1, ISSN 0174-0814, March 1992, p 321. (b) Tu¨rler, A.; Yakushev, A. B.; Ga¨ggeler, H. W.; Jost, D. T.; Scherrer, U.; PSI Annual Report, 1993; p 43. (c) 3d Workshop on Chemie Schwerster Elemente, Mainz, 25-27 May, 1995. (2) Pershina, V.; Fricke, B. J. Phys. Chem. 1994, 98, 6468. (3) Pershina, V.; Fricke, B. J. Phys. Chem. 1995, 99, 144. (4) Canterfold, J. H.; Colton, R. Halides of the Second and Third Row Transition Metals; Wiley: London, 1968. (5) Thermochemical Properties of Inorganic Substances II; Knacke, O., Kubaschewskii, O., Hesselmann, K., Eds.; Springer-Verlag: Berlin, 1991. (6) Jasinski, J. P.; Holt, S. L.; Wood, J. H.; Asprey, L. B. J. Chem. Phys. 1975, 63, 757. (7) Topol, I. A.; Chesnyi, A. S.; Kovba, V. M.; Stepanov, N. F. Theor. Chim. Acta (Berlin) 1982, 61, 369. (8) (a) Rosen, A.; Ellis, D. E. J. Chem. Phys. 1975, 62, 3039. (b) Rosen, A. Int. J. Quantum Chem. 1978, 13, 509. (9) Pershina, V.; Sepp, W.-D.; Fricke, B.; Rosen, A. J. Chem. Phys. 1992, 96, 8367. (10) Pershina, V.; Sepp, W.-D.; Fricke, B.; Kolb, D.; Scha¨del, M.; Ionova, G. V. J. Chem. Phys. 1992, 97, 1116. (11) Pershina, V.; Sepp, W.-D.; Bastug, T.; Fricke, B.; Ionova, G. V. J. Chem. Phys. 1992, 97, 1123. (12) Pershina, V.; Fricke, B. J. Chem. Phys. 1993, 99, 9720. (13) Pershina, V.; Fricke, B. J. Phys. Chem. 1994, 98, 6468. (14) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833. (15) Hobbs, W. E. J. Chem. Phys. 1958, 28, 1220. (16) Iorns, T. V.; Stafford, F. E. J. Am. Chem. Soc. 1966, 4819. (17) Barraclough, C. G.; Stals, J. Aust. J. Chem. 1966, 19, 741. (18) Ward, B. G.; Stafford, F. E. Inorg. Chem. 1968, 7, 2569.

Group 6 Dioxydichlorides MO2Cl2 (M ) Cr, Mo, W) (19) Levanson, W.; Narayanaswamy, R.; Ogden, J. S.; Rest, A. J.; Turff, J. W. J. Chem. Soc., Dalton Trans. 1982, 2009. (20) (a) Zharskii, I. M. Doctoral Thesis, Moscow State University, 1975. (b) Zharskii, I. M.; Zasorin, E. Z.; Spiridonov, V. P.; Novikov, G. I.; Kupreev, V. N. Koord. Khim. 1975, 1, 574. (21) Jampolskii, V. I. Doctoral Thesis, Moscow State University, 1973. (22) Mardsen, C. J.; Hedberg, L.; Hedberg, K. Inorg. Chem. 1982, 21, 1115. (23) Pyykko¨, P.; Desclaux, J.-P. Chem. Phys. 1978, 34, 261. (24) (a) Wagman, D. D.; Evans, W. H.; Parker, V. B.; Schumm, R. H.; Hallow, I.; Bailey, S. M.; Churney, K. L.; Nuttall, R. L. J. Phys. Chem. Ref. Data 1982, 11, Suppl. 2. (b) Chase, Jr.; M. V.; Davies, C. A.; Downey,

J. Phys. Chem., Vol. 100, No. 21, 1996 8751 Jr.; J. R.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. J. Phys. Chem. Ref. Data 1985, 14, Suppl. 1. (25) Ionova, G. V.; Pershina, V.; Zuraeva, I. T.; Suraeva, N. I. SoV. Radiochem. 1995, 37, 307. (26) (a) Shchukarev, S. A.; Suvorov, A. V. Vestn. Leningr. UniV. 16, Ser. Fiz. i Khim. 1961, 87. (b) Shchukarev, S. A.; Suvorov, A. V. Russ. J. Inorg. Chem. 1961, 6, 763. (27) Funaki, K.; Uchimura, K. Denki Kagaku 1962, 30, 106. (28) Shchukarev, S. A.; Novikov, G. I.; Vasil’kova, I. V. Russ. J. Inorg. Chem. 1960, 5, 802.

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