Hexamethyltungsten

Hexamethyltungsten
Other names

Tungsten hexamethyl
Identifiers
CAS number 36133-73-0 Y
ChemSpider 11659456 N
Jmol-3D images Image 1
Properties
Molecular formula C6H18W
Molar mass 274.05 g mol−1
Exact mass 274.091783129 g mol-1
Appearance Red crystalline solid / Vivid red gas
Structure
Molecular shape Trigonal prismatic
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Hexamethyltungsten is the chemical compound W(CH3)6. Classified as an organometallic compound, 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.[1][2]

Contents

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.[1] 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.[3] 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:[4]

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 C3v 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 Oh structure.[5]

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.[6] They predicted that other d0 ML6 species such as [NbMe6]−, [TaMe6]−, 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 D3h or C3v symmetry.[7] 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.[8][4]

As shown in the top figure at right, the ideal or D3h trigonal prism in which all six carbons are equivalent is distorted to the C3v 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.[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 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], [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).[13]

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:[1]

W(CH3)6 → 3 CH4 + (CH2)3 + W

Like many organometallic complexes, WMe6 is destroyed by oxygen. Similarly, acids give methane and unidentified tungsten derivatives, while halogens cause release 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;[14] however, to date it has not been used for this purpose. Rather, tungsten hexafluoride and hydrogen are used instead.[15]

Safety considerations

Serious explosions have been reported as a result of working with W(CH3)6, even in the absence of air.[16][5]

See also

References

  1. ^ a b c Shortland, A. J.; Wilkinson, G. (1973). "Preparation and properties of hexamethyltungsten". J. Chem. Soc., Dalton Trans. (8): 872–876. doi:10.1039/DT9730000872. 
  2. ^ Koutsospyros, A.; Braida, W.; Christodoulatos, C.; Dermatas D.; N. Strigul, N. (2006). "A review of tungsten: From environmental obscurity to scrutiny". Journal of Hazardous Materials 136 (1): 1–19. doi:10.1016/j.jhazmat.2005.11.007. PMID 16343746. 
  3. ^ Galyer, A. L.; Wilkinson, G. (1976). "New synthesis of hexamethyltungsten(VI). The octamethyltungstate-(VI) lon". J. Chem. Soc., Dalton Trans. (21): 2235. doi:10.1039/DT9760002235. 
  4. ^ a b Kleinhenz, S.; Pfennig, V.; Seppelt, K.; (1998). "Preparation and Structures of [W(CH3)6], [Re(CH3)6], [Nb(CH3)6]-, and [Ta(CH3)6]-". Chem. Eur. J. 4 (9): 1687. doi:10.1002/(SICI)1521-3765(19980904)4:9<1687::AID-CHEM1687>3.0.CO;2-R. 
  5. ^ a b Green, J. C.; Lloyd, D. R.; Galyer, L.; Mertis, K.; Wilkinson, G. (1978). "Photoelectron spectra of some transition metal alkyls and oxoalkyls". J. Chem. Soc., Dalton Trans. (10): 1403. doi:10.1039/DT9780001403. 
  6. ^ Morse, P. M.; Girolami, G. S. (1989). "Are d0 ML6 complexes always octahedral? The x-ray structure of trigonal-prismatic [Li(tmed)]2[ZrMe6]". J. Am. Chem. Soc. 111 (11): 4114. doi:10.1021/ja00193a061. 
  7. ^ Haalan, A.; Hammel, A.; Rydpal, K.; Volden, H. V. (1990). "The coordination geometry of gaseous hexamethyltungsten is not octahedral". J. Am. Chem. Soc. 112 (11): 4547–4549. doi:10.1021/ja00167a065. 
  8. ^ Seppelt, K.; Pfennig, V. (1996). "Crystal and Molecular Structures of Hexamethyltungsten and Hexamethylrhenium". Science 271 (5249): 626. doi:10.1126/science.271.5249.626. 
  9. ^ Seppelt, Konrad (2003). "Nonoctahedral Structures". Accounts of Chemical Research 36 (2): 147–153. doi:10.1021/ar020052o. PMID 12589700. |author=Seppelt, K.|journal=Accounts of Chemical Research|title=Nonoctahedral Structures|year=2003|volume=36|pages= 147–153. doi: 10.1021/ar020052o}}
  10. ^ Kaupp, M. (1998). "The Nonoctahedral Structures of d0, d1, and d2 Hexamethyl Complexes". Chemistry - A European Journal 4 (9): 1678–86. doi:10.1002/(SICI)1521-3765(19980904)4:9<1678::AID-CHEM1678>3.0.CO;2-N. 
  11. ^ Landis, C. K.; Cleveland, T.; Firman, T. K. (1995). "Making sense of the shapes of simple metal hydrides". J. Am. Chem. Soc. 117 (6): 1859–1860. doi:10.1021/ja00111a036. 
  12. ^ Landis, C. K.; Cleveland, T.; Firman, T. K. (1996). "Structure of W(CH3)6". Science 272 (5259): 182–183. doi:10.1126/science.272.5259.182b. 
  13. ^ Housecroft, C. E.; Sharpe, A. G. (2004). Inorganic Chemistry (2nd ed.). Prentice Hall. ISBN 978-0130399137. 
  14. ^ Matsumoto, S.; Ikeda, O.; Ohmi, K. (Canon K. K., Japan) (1991). "Eur. Pat. Appl.". 
  15. ^ Kirss, R. U.; Meda, L. (1998). "Chemical vapor deposition of tungsten oxide". Applied Organometallic Chemistry 12 (3): 155–160. doi:10.1002/(SICI)1099-0739(199803)12:3<155::AID-AOC688>3.0.CO;2-Z. 
  16. ^ Mertis, K.; Galyer, L.; Wilkinson, G. (1975). "Permethyls of tantalum, tungsten and rhenium: a warning". Journal of Organometallic Chemistry 97 (3): C65. doi:10.1016/S0022-328X(00)89324-9.