Gallane

Structure of monomeric GaH3

I.R spectroscopic studies indicate that monomeric GaH₃ has a trigonal planar structure Theoretical Ga-H bond lengths have been calculated as being in the range 155.7 pm to 158.7 pm

Monomeric GaH₃ dimerises in the vapor phase to form Ga₂H₆, digallane(6) and the enthalpy change associated with the gas phase dissociation reaction Ga₂H₆ → 2GaH₃ has been experimentally estimated as 59 +/- 16 kJ mol⁻¹.

Chemical properties

As GaH₃ cannot be prepared or isolated readily reactions involving GaH₃ either use the dimer, Ga₂H₆, digallane(6) or adducts of GaH₃ for example L.GaH₃ where L is a monodentate ligand.

GaH3 adducts

The production of adducts can proceed via the direct reaction of digallane(6) or more often due to the thermal fragility of digallane(6) (which decomposes to gallium metal and hydrogen above -20 °C) using a tetrahydridogallate salt as a starting point (e.g. LiGaH₄) or alternatively via ligand displacement from an existing adduct. Examples are:-

Ga₂H₆ + 2NMe₃ → (NMe₃)2GaH₃ (-95°C) LiGaH₄ + Me₃NHCl → LiCl + H₂+ Me₃N^.GaH₃ Me₂NH + Me₃N^.GaH₃ → Me₂NH. GaH₃ + Me₃N

Many adducts have been prepared. There are a number of typical structures with neutral adducts (L = monodentate ligand, L-L is bidentate):-

L.GaH₃ (1:1 complex with monodentate ligand giving 4 coordinate gallium) L₂.GaH₃ (2:1 complex with monodentate ligand giving 5 coordinate gallium) H₃Ga.L-L.GaH₃ (1:2 complex with a bidentate ligand with two 4 coordinate gallium atoms) L'H₃Ga.L-L.GaH₃L' (complex with monodentate and bidentate ligands with two 5 coordinate gallium atoms) LGaH₂(μ-H)₂GaH₂L ( 2:2 hydrogen bridged complex.) (-L-LGaH₃-)_n a 1:1 complex with a bidentate ligand forming a polymeric structure.

In comparison to alane (AlH₃) with similar ligands, gallane tends to adopt lower coordination numbers. Also whilst N donor ligands form stronger bonds to alumane than phosphines the reverse is typically true for gallane. The monomeric structure of Me₃N^.GaH₃ has been confirmed in both the gas and solid phases. In this regard, the 1:1 adduct contrasts with the corresponding alane complex, Me₃N^.AlH₃ which in the solid is dimeric with bridging hydrogen atoms.

Acidity

The gallium centre in the gallyl group (-GaH
2) in gallanes such as gallane can accept an electron-pair donating ligand into the molecule by adduction, as mentioned above:

[GaH
3] + L → [GaH
3L]

Because of this acceptance of the electron-pair donating ligand (L), gallane has Lewis-acidic character.

Gallane does not form stable aqueous solutions due to hydrolysis.

GaH
3 + 3 H
2O → Ga(OH)
3 + 3 H
2

Solute properties

Gaseous gallane is a hydrophilic (non-polar) aprotic solute. It dissolves in polar compounds such as tetramethylethylenediamine, from which it can be crystallised as gallane—N,N,N′,N′-tetramethylethane-1,2-diamine (1/1).

Other chemical reactions

Upon treatment with a standard base, it converts to a metal tetrahydroxygallanuide (the anion Ga(OH)₄⁻) and hydrogen gas. With strong bases, it can be deprotonated to give GaH−
2. Reduction of gallane gives gallium metal. Upon treatment with a standard acid, it converts to a gallium(3+) salt and hydrogen gas. Oxidation of gallane gives Ga(OH)₃, gallium(III) hydroxide. Unsolvated gallane is in chemical equilibrium with digallane(6), being the dominant species with increasing temperature. Due to this equilibrium, gallane and digallane(6) are often considered to be chemically equivalent. Reactions requiring gallane as opposed to digallane(6), must be carried out in solution. Common solvents include tetrahydrofuran, and diethyl ether.