The digallane molecule, Ga2H6: experimental update giving an improved structure and estimate of the enthalpy change for the reaction Ga2H6(g) → 2GaH3(g)

Abstract

Improved methods of analysis and new quantum chemical calculations have been applied to the results of earlier gas-phase electron diffraction (GED) studies of digallane to give what is judged to be the most realistic structure available to date. The principal distances (r_a3,1 in pm) and interbond angles (∠_a3,1 in deg) are as follows (t = terminal, b = bridging): r(Ga⋯Ga) 254.9(2), r(Ga−H_t) 155.0(6), r(Ga−H_b) 172.3(6), ∠Ga−H_b−Ga 95.4(5), and ∠H_t−Ga−H_t 128.6(9). Scrutiny of the IR spectra of solid Ar matrices doped with the vapour above solid samples of gallane at temperatures in the range 190-220 K reveals the presence of not only Ga₂H₆ as the major component, but also a significant fraction of the monomer GaH₃. Analysis of the relative proportions of the two molecules evaporating from the solid at different temperatures has led to a first experimental estimate of 59 ± 16 kJ mol⁻¹ for the enthalpy change associated with the reaction Ga₂H₆(g) → 2GaH₃(g). Together with a value of 52 kJ mol⁻¹ delivered by fresh calculations at the MP2 level, this implies that the stability of the dimer with respect to dissociation has been overrated by earlier theoretical treatments.