Hexamethyl tungsten

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Hexamethyl tungsten
Hexamethyl tungsten Ball-and-stick model of hexamethyl tungsten Polyhedral model of hexamethyl tungsten
General
Systematic name Hexamethyl tungsten
Aliases Tungsten hexamethyl
Molecular formula (CH3)6W
Molar mass 274.06 g/mol
Appearance red crystalline solid
CAS number [36133-73-0]
Properties
Melting point 30 °C (303 K)
Structure
Molecular shape trigonal prismatic

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°. 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].

Contents

[edit] Synthesis

Hexamethyl tungsten was first reported synthesized in 1973 by Wilkinson and Shortland.[1] Sir Geoffrey Wilkinson was awarded a share of the 1973 Nobel Prize in Chemistry for his work on organometallic compounds.

Methyl-lithium in diethyl ether is added to a suspension of tungsten hexachloride in diethyl ether at -20 °C. The ether is removed at 0 °C in vacuo and then extracted with petroleum to give a dark red solution. Removal of the petroleum at -20 °C in vacuo and sublimation to a probe at -10 °C gives the red crystal solid.

In 1976, Wilkinson and Galyer reported on an improved synthesis of W(CH3)6 using WCl6 and Al(CH3)3. [3] The initial method, described above, is quoted as frequently giving poor or sometimes zero yields. The stoichiometry of the improved synthesis is as follows:

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


To a suspension of tungsten hexachloride in isopentane at -70 °C, six molar equivalents of trimethylaluminum are added via syringe over the course of 15 minutes with vigorous stirring. The solution is allowed to slowly warm to room temperature with continued stirring, until the solution reaches ca. 20 °C, at which point it is recooled to -70 °C. To the cooled solution, an excess of dried trimethylamine is added and the resultant slurry is stirred rapidly until the exothermic reaction subsides. The AlMe2Cl-NMe3 precipitate is filtered off to yield a deep red-orange filtrate, which is then concentrated in vacuo at -10 °C. Additional recooling and filtration removes more of the unwanted byproducts.

[edit] Molecular Geometry

W(CH3)6 has a distorted trigonal prismatic coordination geometry with C3v 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 instable, 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 Oh 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 D3h 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]

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]. 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 (D3h symmetry) include [ReMe6] (d1), [TaMe6]- (d0), and the aforementioned [ZrMe6]2- (d0).[11]

[edit] 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



[edit] 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.

[edit] 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;[12] however, to date it has not been used for this purpose. WF6(g) + H2(g) is used instead.[13]

[edit] 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.[14][15]

[edit] See also

[edit] References

  1. ^ a b c 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 Materials 2006, 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. Science 1996, 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 Research 2003, 36(2), 147-153
  10. ^ Kaupp, M. Chemistry - A European Journal 1998, 4(9), 1678-86
  11. ^ Housecroft, C. E.; Sharpe, A. G. (2005) Inorganic Chemistry 2nd ed. Pearson Education Limited: England.
  12. ^ Matsumoto, S.; Ikeda, O.; Ohmi, K. (Canon K. K., Japan). Eur. Pat. Appl. 1991
  13. ^ Kirss, R. U.; Meda, L. Applied Organometallic Chemistry 1998, 12(3), 155–160
  14. ^ Mertis, K.; Galyer, L.; Wilkinson, G. Journal of Organometallic Chemistry 1975, 97(3), C65.
  15. ^ Green, J.C.; Lloyd, D. R.; Galyer, L.; Mertis, K.; Wilkinson, G. Inorganic Chemistry 1978, (10), 1403-7.

[edit] External links

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