Group 3 tetrahydroborates. Part 4. The molecular structure of hydridogallium bis(tetrahydroborate) in the gas phase as determined by electron diffraction

Abstract

Gaseous Ga(BH₄)₂H, as studied by electron diffraction, appears to consist of monomeric HGa[(µ-H)₂BH₂]₂ molecules with five-fold co-ordination of the gallium atom, a single terminal (t) Ga–H bond, and two doubly bridged (b) tetrahydroborate groups. The primary features of the structure involve the dimensions: r(Ga–B) 217.2(0.5), r(Ga–H)(average) 177.4(1.7), and r(B–H)(average) 127.7(1.4) pm; B–Ga–B 112.2(1.5)°(distances correspond to r_a; figures in parentheses are the estimated standard deviations of the last digits). Results are given for refinements based on a structural model with C_2v symmetry imposed, but a significantly better fit to the experimental data is achieved if this constraint is relaxed so that the Ga(µ-H)₂B moieties assume an unsymmetrical form with r(Ga–H_b) 176.2(1.5) and 189.1(2.5) pm and r(B–H_b) 125.0(8.1) and 145.6(3.3) pm. The five hydrogen atoms directly bound to the gallium atom form a slightly distorted rectangularly based pyramid. The features of the structure are collated with those of other hydridogallium and tetrahydroborate derivatives.