Digallane

Digallane
IUPAC name digallane(6)
Other names Di-μ-hydrido-tetrahydridodigallium Gallane dimer
Identifiers
CAS number	12140-58-8
Properties
Molecular formula	Ga2H6
Molar mass	145.494 g/mol
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Infobox references

Digallane is a chemical compound of gallium and hydrogen with the formula Ga₂H₆. It is the dimer of the monomeric compound gallane. The eventual preparation of the pure compound, reported in 1989,^[1][2] was hailed as a "tour de force."^[3] Digallane had been reported as early as 1941 by Wiberg;^[4] however, this claim could not be verified by later work by Greenwood and others.^[5]

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 and thus formulated as (H₂Ga(μ-Cl))₂) was prepared via the hydrogenation of gallium trichloride, GaCl₃, with Me₃SiH. This step was followed by a further reduction with LiGaH₄, solvent free, at −23 °C, to produce digallane, Ga₂H₆ in low yield.

Ga₂Cl₆ + 4 Me₃SiH → (H₂GaCl)₂ + 4 Me₃SiCl

1/2 (H₂GaCl)₂ + LiGaH₄ → Ga₂H₆ + LiCl

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

Structure and bonding

Electron diffraction measurements of the vapour at 255 K established that digallane is structurally similar to diborane with 2 bridging hydrogen atoms^[2] (so-called three-center two-electron bonds). The terminal Ga—H bond length is 152 pm, the Ga—H bridging is 171 pm and the Ga—H—Ga angle is 98°. The Ga—Ga distance is 258 pm. The ¹H NMR spectrum of a solution of digallane in toluene shows two peaks attributable to terminal and bridging hydrogen atoms.^[2]

In the solid state, digallane appears to adopt a polymeric or oligomeric structure. The vibrational spectrum is consistent with tetramer (i.e. (GaH₃)₄).^[2] The vibrational data indicate the presence of terminal hydride ligands. In contrast, the hydrogen atoms are all bridging in α-alane, a high-melting, relatively stable polymeric form of aluminium hydride wherein the aluminium centers are 6-coordinated.

Reactions

Digallane decomposes at ambient temperatures:

Ga₂H₆ → 2 Ga + 3 H₂

The reactions with Lewis bases are similar to those of diborane, but the larger size of gallium is indicated by its ability to form 2:1 adducts. Thus, with trimethylamine both 1:1 and 2:1 adducts are formed (i.e. Me₃N·GaH₃ and (Me₃N)₂·GaH₃, respectively. With phosphine a 1:1 adduct H₃P·GaH₃ is formed. The trimethylamine adducts have been known for some time. ^[6][7] These adducts are 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 monomeric structure of Me₃N^.GaH₃ has been confirmed in both the gas and solid phases. In this regard, the 2:1 adduct contrasts with the corresponding alane complex, Me₃N^.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 Trans. (14): 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, (CH₃N)₂GaH₃". 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·GaH_3<: 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, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0080379419.