Digallane (systematically named digallane(6)) is an
inorganic compound
In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemi ...
with the
chemical formula
In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...
(also written or ). It is the
dimer of the monomeric compound
gallane. The eventual preparation of the pure compound, reported in 1989,
was hailed as a "tour de force." Digallane had been reported as early as 1941 by Wiberg; however, this claim could not be verified by later work by Greenwood and others.
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, (H
2GaCl)
2 (containing
bridging chlorine atoms and thus formulated as (H
2Ga(μ-Cl))
2) was prepared via the hydrogenation of
gallium trichloride
Gallium trichloride is the chemical compound with the formula GaCl3. Solid gallium trichloride exists as a dimer with the formula Ga2Cl6. It is colourless and soluble in virtually all solvents, even alkanes, which is truly unusual for a metal h ...
, GaCl
3, with
Me3SiH. This step was followed by a further reduction with LiGaH
4, solvent free, at −23 °C, to produce digallane, Ga
2H
6 in low yield.
:Ga
2Cl
6 + 4 Me
3SiH → (H
2GaCl)
2 + 4 Me
3SiCl
:1/2 (H
2GaCl)
2 + LiGaH
4 → Ga
2H
6 + 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
(so-called
three-center two-electron bond
A three-center two-electron (3c–2e) bond is an electron-deficient chemical bond where three atoms share two electrons. The combination of three atomic orbitals form three molecular orbitals: one bonding, one ''non''-bonding, and one ''anti''- ...
s). 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
Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon. It is a colorless, water-insoluble liquid with the smell associated with paint thinners. It is a mono-substituted benzene derivative, consisting of a methyl group (CH3) ...
shows two peaks attributable to terminal and bridging hydrogen atoms.
In the solid state, digallane appears to adopt a polymeric or oligomeric structure. The vibrational spectrum is consistent with tetramer (i.e. (GaH
3)
4).
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:
: Ga
2H
6 → 2 Ga + 3 H
2
References
{{Hydrides by group
Inorganic compounds
Gallium compounds
Metal hydrides
Reducing agents
Substances discovered in the 1980s