Hexaméthyltungstène

Hexamethyltungsten is the chemical compound W(CH3)6 also written WMe6. Classified as a transition metal alkyl complex, hexamethyltungsten 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. 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) W(CH3)6 adopts a distorted trigonal prismatic geometry with C3v symmetry for the WC6 framework and C3 symmetry including the hydrogen atoms. The structure (excluding the hydrogen atoms) can be thought of as consisting of a central atom, capped on either side by two eclipsing sets of three carbon atoms, with one triangular set slightly larger but also closer to the central atom than the other. 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 Oh structure. The octahedral assignment remained for nearly 20 years until 1989 when Girolami and Morse showed that [Zr(CH3)6]2- was trigonal prismatic as indicated by X-ray crystallography.