Digallane
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Digallane (systematically named digallane(6)) is an inorganic compound with the chemical formula Template:Chem2 (also written Template:Chem2 or Template:Chem2). 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] This compound is a colorless gas that decomposes above 0 °C.
Preparation
A two-stage approach proved to be the key to successful synthesis of pure digallane. Firstly the dimeric monochlorogallane, Template:Chem2 (containing bridging chlorine atoms and thus formulated as (Template:Chem2) was prepared via the hydrogenation of gallium trichloride, Template:Chem2, with trimethylsilane, Template:Chem2. This step was followed by a further reduction with Template:Chem2 (lithium tetrahydrogallate), solvent free, at −23 °C, to produce digallane, Template:Chem2 in low yield.
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. Template:Chem2).[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. Digallane decomposes at ambient temperatures:
References
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