Hexamethyltungsten

Synthesis

Hexamethyltungsten was first reported in 1973 by Wilkinson and Shortland, who described its preparation by the reaction of methyllithium with tungsten hexachloride in diethyl ether. The synthesis was motivated in part by previous work which indicated that tetrahedral methyl transition metal compounds are thermally unstable, in the hopes that an octahedral methyl compound would prove to be more robust. In 1976, Wilkinson and Galyer disclosed an improved synthesis using trimethylaluminium in conjunction with trimethylamine, instead of methyllithium. The stoichiometry of the improved synthesis is as follows:

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

Alternatively, the alkylation can employ dimethylzinc:

WX₆ + 3 Zn(CH₃)₂ → W(CH₃)₆ + 3 ZnX₂ (X = F, Cl)

Molecular geometry

W(CH₃)₆ adopts a distorted trigonal prismatic geometry with D_3h symmetry. The trigonal prismatic geometry is unusual in that the vast majority of six-coordinate organometallic compounds adopt octahedral molecular geometry. In the initial report, the IR spectroscopy results were interpreted in terms of an octahedral structure. In 1978, a study using photoelectron spectroscopy appeared to confirm the initial assignment of an O_h structure.

The octahedral assignment remained for nearly 20 years until 1989 when Girolami and Morse showed that [Zr(CH
3)
6]2−
was trigonal prismatic as indicated by X-ray crystallography. They predicted that other d⁰ ML₆ species such as [Nb(CH
3)
6]−
, [Ta(CH
3)
6]−
, and W(CH₃)₆ would also prove to be trigonal prismatic. This report prompted other investigations into the structure of W(CH₃)₆. Using gas-phase electron diffraction, Volden et al. confirmed that W(CH₃)₆ is indeed trigonal prismatic structure with either D_3h or C_3v symmetry. In 1996, Seppelt et al. reported that W(CH₃)₆ had a strongly distorted trigonal prismatic coordination geometry based on single-crystal X-ray diffraction, which they later confirmed in 1998.

As shown in the top figure at right, the ideal or D_3h trigonal prism in which all six carbon atoms 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 an effect known as a second-order Jahn-Teller distortion. 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.

The history of 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], [NbMe
6]−
, and [TaPh
6]−
. All are d⁰ complexes. Some 6-coordinate complexes with regular trigonal prismatic structures (D_3h symmetry) include [ReMe₆] (d¹), [TaMe
6]−
(d⁰), and the aforementioned [ZrMe
6]2−
(d⁰).

Reactivity and potential uses

At room temperature, hexamethyltungsten decomposes, releasing methane and trace amounts of ethane. The black residue 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:

W(CH
3)
6 → 3 CH
4 + (CH
2)
3 + W

Like many organometallic complexes, WMe₆ is destroyed by oxygen. Similarly, acids give methane and unidentified tungsten derivatives, while halogens give the methyl halide and leave the tungsten halide.

A patent application was submitted in 1991 suggesting the use of W(CH₃)₆ in the manufacture of semiconductor devices for chemical vapor deposition of tungsten thin films; however, to date it has not been used for this purpose. Rather, tungsten hexafluoride and hydrogen are used instead.

Safety considerations

Serious explosions have been reported as a result of working with W(CH₃)₆, even in the absence of air.