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

Abstract

Gaseous methylaluminium bis(tetrahydroborate), Al(BH₄)₂Me, as studied by electron diffraction, appears to be monomeric with a planar CAlB₂ skeleton. The structure involves five-fold co-ordination of the aluminium atom, each of the tetrahydroborate groups functioning as a bidentate ligand; the planes containing the Al(µ-H)₂B units evidently depart from being normal to the plane of the CAlB₂ skeleton by ca. 5°, thereby reducing the symmetry of the CAl(BH₄)₂ moiety from C_2v to C₂. Other salient parameters (estimated standared deviations in parentheses) are: r_a(Al–C) 194.1(0.9), r_a(Al–B) 215.2(1.6), r_a(Al–H_b) 182.0(0.9), r_a(B–H_b) 124, r_a(B–H_t) 120 pm; B–Al–B 121.5(0.7), H_b–Al–H_b 70.3(0.4)°(t = terminal, b = bridging). The features of the structure are compared with those of other molecules in the series Al(BH₄)_3–nMe_n(n= 0–3).