Hexamethyl tungsten

Hexamethyl tungsten is the chemical compound W(CH₃)₆. Classified as an organometallic compound, hexamethyl tungsten is an air-sensitive, red, crystalline solid at room temperature; however, it is extremely volatile and sublimes at −30 °C. Owing to its six methyl groups it is extremely soluble in petroleum, aromatic hydrocarbons, ethers, carbon disulfide, and carbon tetrachloride.^[1] The compound is unknown in nature.^[2].

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Synthesis

Hexamethyl tungsten was first reported synthesized in 1973 by Wilkinson and Shortland by reacting methyl lithium with tungsten hexachloride in diethyl ether.^[1] Unfortunately, this method is quoted as frequently giving poor or sometimes zero yields.^{[citation needed]} Geoffrey Wilkinson was awarded a share of the 1973 Nobel Prize in Chemistry for his work on organometallic compounds.

In 1976, Wilkinson and Galyer reported on an improved synthesis of W(CH₃)₆ using trimethylaluminium instead of methyl lithium, and the use of trimethylamine.^[3] The stoichiometry of the improved synthesis is as follows:

WCl₆ + 6Al(CH₃)₃ → W(CH₃)₆ + 6Al(CH₃)₂Cl

Molecular Geometry

W(CH₃)₆ has a distorted trigonal prismatic geometry with C_3v symmetry. The trigonal prismatic geometry is unusual, in that the vast majority of six-coordinate organometallics have octahedral molecular geometry. W(CH₃)₆ has an interesting history in terms of identifying its molecular structure. When it was initially synthesized, Wilkinson et al believed that W(CH₃)₆ would have an octahedral structure.^[1] In fact, the initial motive for synthesizing the compound was based on previous work with tetrahedral methyl transition metal compounds, which are thermally unstable, in the hopes that an octahedral methyl compound would prove to be more stable. In the initial report, the IR spectroscopy results were consistent with an octahedral structure. In 1978, a study using photoelectron spectroscopy confirmed the initial O_h assignment.^[4]

The octahedral assignment remained for nearly 20 years after its discovery. In 1989, Girolami and Morse reported that [Zr(CH₃)₆]^2- did not have octahedral geometry as previously believed, but rather trigonal prismatic geometry based on their X-ray crystallography results.^[5] They predicted that other d⁰ ML₆ species such as [NbMe₆]^-, [TaMe₆]^-, and W(CH₃)₆ would also prove to be trigonal prismatic. This prompted others to investigate the structure of W(CH₃)₆ in more detail, and in 1990 Volden et al made the first confirmation that W(CH₃)₆ did indeed have trigonal prismatic structure with D_3h or C_3v symmetry based on their results from gas-phase electron diffraction.^[6] In 1996, Seppelt et al. reported that W(CH₃)₆ had a strongly distorted trigonal prismatic coordination geometry from their work with single-crystal X-ray diffraction, which they later confirmed in 1998.^[7][8]

As shown in the top figure at right, the ideal or D_3h trigonal prism in which all six carbons are equivalent is distorted to the C_3v structure observed by Seppelt et al. by opening up one triplet of methyl groups (upper triangle) to wider C-W-C angles (94-97°) with slightly shorter C-W bond lengths, while closing the other triplet (lower triangle) to 75-78° with longer bond lengths.

Deviation from octahedral geometry can be ascribed to the electronic configuration of the central atom, tungsten in this case, and an effect known as Jahn-Teller distortion.^[9][10]. In 1995, before the work of Seppelt and Pfennig, Landis and coworkers had already predicted a distorted trigonal prismatic structure based on valence bond theory and VALBOND calculations.^{[11] [12]} The interested reader is encouraged to read Nonoctahedral Structures^[9] by Konrad Seppelt for a more rigorous treatment. The history of elucidating the structure of W(CH₃)₆ illustrates an inherent difficulty in interpreting spectral data for new compounds: initial data may not provide reason to believe the structure deviates from a presumed geometry based on significant historical precedence, but there is always the possibility that the initial assignment will prove to be incorrect. Prior to 1989, there was no reason to suspect that ML₆ compounds were anything but octahedral, yet new evidence and improved characterization methods suggested that perhaps there were exceptions to the rule, as evidenced by the case of W(CH₃)₆. These discoveries helped to spawn re-evaluation of the theoretical considerations for ML₆ geometries.

Other 6-coordinate complexes with distorted trigonal prismatic structures include [MoMe₆], [NbMe₆]^-, and [TaPh₆]^-. All are d⁰ complexes. Some 6-coordinate complexes with regular trigonal prismatic structures (D_3h symmetry) include [ReMe₆] (d¹), [TaMe₆]^- (d⁰), and the aforementioned [ZrMe₆]^2- (d⁰).^[13]

Stability

At room temperature, hexamethyl tungsten will decompose to give off methane and trace amounts of ethane. The black residue that remains is purported to contain polymethylene and tungsten, but the decomposition of W(CH₃)₆ to form tungsten metal is highly unlikely. The following equation is the approximate stoichiometry proposed by Wilkinson and Shortland:^[1]

W(CH₃)₆ → 3CH₄ + (CH₂)₃ + W

Reactivity

• Oxygen: W(CH₃)₆ is spontaneously flammable in air.
• Acidic protons: W(CH₃)₆ reacts with compounds containing acidic protons to release methane.
• Halogens: W(CH₃)₆ reacts with halogens such as bromine and iodine to release the methyl halide and leave the tungsten halide.

Industrial uses

A patent application was submitted in 1991 suggesting the use of W(CH₃)₆ in the manufacture of semiconductor devices during the chemical vapor deposition of tungsten thin films;^[14] however, to date it has not been used for this purpose. WF₆(g) + H₂(g) is used instead.^[15]

Safety and pollution considerations

W(CH₃)₆ should be handled with extreme care due to the risk of explosions, even in the absence of air. Serious explosions have been reported as a result of working with the compound.^[16][17]

See also

• Organometallic Compounds
• Tungsten Compounds
• Tungsten hexachloride
• Jahn-Teller Effect
• Sir Geoffrey Wilkinson

References

1. ^ ^{a b c d} Shortland, A. J.; Wilkinson, G.J. Chem. Soc., Dalton Trans.1973, 872-876.
2. ^ Koutsospyros, A.; Braida, W.; Christodoulatos, C.; Dermatas D.; N. Strigul, N.;Journal of Hazardous Materials2006,136, 1-19
3. ^ Galyer, A. L.; Wilkinson, G.J. Chem. Soc., Dalton Trans.1976, 2235-8
4. ^ Green, J. C.; Lloyd, D. R.; Galyer, L.; Mertis, K.; Wilkinson, G.J. Chem. Soc., Dalton Trans.1978, 1403-7
5. ^ Morse, P. M.; Girolami, G. S.J. Am. Chem. Soc.1989,111, 4114-6
6. ^ Haalan, A.; Hammel, A.; Rydpal, K.; Volden, H. V.J. Am. Chem. Soc.1990,112, 4547-4549.
7. ^ Seppelt, K.; Pfennig, V.Science1996,271, 626-8.
8. ^ Kleinhenz, S.; Pfennig, V.; Seppelt, K.;Chem. Eur. J.1998,4, 1687-91.
9. ^ ^{a b} Seppelt, K.Accounts of Chemical Research2003,36(2), 147-153
10. ^ Kaupp, M.Chemistry - A European Journal1998,4(9), 1678-86
11. ^ Landis, C. K.; Cleveland, T.; Firman, T. K. Making sense of the shapes of simple metal hydrides. J. Am. Chem. Soc. 1995, 117, 1859-1860.
12. ^ Landis, C. K.; Cleveland, T.; Firman, T. K. Structure of W(CH₃)₆. Science 1996, 272, 182-183.
13. ^ Housecroft, C. E.; Sharpe, A. G.(2005)Inorganic Chemistry 2^nd ed. Pearson Education Limited: England.
14. ^ Matsumoto, S.; Ikeda, O.; Ohmi, K. (Canon K. K., Japan).Eur. Pat. Appl.1991
15. ^ Kirss, R. U.; Meda, L.Applied Organometallic Chemistry1998,12(3), 155–160
16. ^ Mertis, K.; Galyer, L.; Wilkinson, G.Journal of Organometallic Chemistry1975,97(3), C65.
17. ^ Green, J.C.; Lloyd, D. R.; Galyer, L.; Mertis, K.; Wilkinson, G.Inorganic Chemistry1978,(10), 1403-7.