Digallane

Digallane is a chemical compound of gallium and hydrogen with the formula Ga₂H₆. The eventual preparation of the pure compound, reported in 1989, ^{[1] [2]} was hailed by N N Greenwood as a "tour de force".^[3] Historically digallane had been reported as early as 1941 by E. Wiberg^[4], however this was not verified by later work by N.N Greenwood and others.^[5]

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Preparation

A two stage approach proved to be the key to successful synthesis of pure digallane. Firstly the dimeric monochlorogallane, (H₂GaCl)₂ (containing bridging chlorine atoms this can be formulated as (H₂Ga(μ-Cl))₂) was prepared via the hydrogenation of gallium trichloride, GaCl₃ with Me₃SiH. This was followed by a further reduction with LiGaH₄, solvent free, at −23°C to produce digallane, Ga₂H₆, in low yield.

Ga₂Cl₆ + 2Me₃SiH → (H₂GaCl)₂ + 2Me₃SiCl

(H₂GaCl)₂ + LiGaH₄ → Ga₂H₆ + LiCl

Digallane is volatile and condenses at −50°C to give a white solid.

Structure and bonding

Electron diffraction measurements of the vapour at 255°K show the structure of digallane to be similar to that of diborane with 2 bridging hydrogen atoms.^[2] The terminal Ga-H bond length is 152pm, the Ga-H bridging is 171 pm and the Ga-H-Ga angle is 98°. The Ga-Ga distance is 258pm. The NMR spectrum of a digallane solution in toluene shows to peaks attributable to terminal and bridging hydrogen atoms.^[2]
The solid appears to be polymeric, and on the basis of its vibrational spectrum is possibly tetrameric (i.e. GaH₃)₄.^[2] There are terminal, non-bridging, hydrogen atoms which contrasts with α-alane, a high melting, relatively stable polymeric form of aluminium hydride, where all hydrogen atoms are equivalent and the aluminium atoms are 6 coordinate.

Reactions

Digallane decomposes at ambient temperatures:

2GaH₃ → 2Ga + 3H₂

The reactions with Lewis bases are similar to those of diborane. With trimethylamine 1:1 and 2:1 adducts are formed i.e. NMe₃.GaH₃ (NMe₃)₂.GaH₃. With phosphine a 1:1 adduct PH₃.GaH₃. The trimethylamine adducts have been known for some time. ^{[6] [7]} They were prepared by the reaction of LiGaH₄ with trimethylammonium chloride, Me₃NHCl, the 2:1 adduct is formed at low temperatures:

LiGaH₄ + Me₃NHCl → LiCl + H₂+ Me₃N.GaH₃

The structure of Me₃N.GaH₃ was reexamined and it has been confirmed to be monomeric in both the gas and solid phases. In this it contrasts with the corresponding alane adduct, NMe₃.AlH₃ which in the solid is dimeric with bridging hydrogen atoms. ^[8] A range of other 1:1 adducts have been prepared and their stabilities determined. ^[9]

References

1. ^ Anthony J. Downs, Michael J. Goode, and Colin R. Pulham (1989). "Gallane at last!". Journal of the American Chemical Society 111 (5): 1936 - 1937. doi:10.1021/ja00187a090.
2. ^ ^{a b c d} Pulham C.R., Downs A.J., Goode M.J, Rankin D.W.H. Roberson H.E. (1991). "Gallane: Synthesis, Physical and Chemical Properties, and Structure of the Gaseous Molecule Ga₂H₆ As Determined by Electron Diffraction". Journal of the American Chemical Society 113 (14): 5149 - 5162. doi:10.1021/ja00014a003.
3. ^ N.N. Greenwood (2001). "Main group element chemistry at the millennium". J Chem Soc Dalton Transactions: 2055 - 2066. doi:10.1039/b103917m.
4. ^ Wiberg E.; Johannsen T. (1941). "Über einen flüchtigen Galliumwasserstoff der Formel Ga₂H₆ und sein Tetramethylderivat". Naturwissenschaften 29 (21): 320. doi:10.1007/BF01479551.
5. ^ Shriver, D. F.; Parry, R. W.; Greenwood, N. N.; Storr, A,; Wallbridge, M. G. H. (1963). "Some Observations Relative to Digallane". Inorg. Chem. 2 (4): 867 - 868. doi:10.1021/ic50008a053.
6. ^ N. N. Greenwood, A. Storr, and M. G. H. Wallbridge (1963). "Trimethylamine Adducts of Gallane and Trideuteriogallane Gallane, (CH₃)₃NGaH₃". Inorg. Chem. 2 (5): 1036 - 1039. doi:10.1021/ic50009a036.
7. ^ D. F. Shriver, C. E. Nordman (1963). "The Crystal Structure of Trimethylamine Gallane, (CH₃)₃NGaH₃". Inorg. Chem. 2 (6): 1298 - 1300. doi:10.1021/ic50010a047.
8. ^ Brain P.T., Brown H.E., Downs A.J., Greene T.M., Johnsen E, Parsons S, Rankin D.W.H, Smart B.A, Tang C.Y. (1998). "Molecular structure of trimethylamine–gallane, Me₃N·GaH₃: ab initio calculations, gas-phase electron diffraction and single-crystal X-ray diffraction studies". J. Chem. Soc., Dalton Trans (21): 3685. doi:10.1039/a806289g.
9. ^ Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements, 2nd Edition, Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.