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 C2v to C2. Other salient parameters (estimated standared deviations in parentheses) are: ra(Al–C) 194.1(0.9), ra(Al–B) 215.2(1.6), ra(Al–Hb) 182.0(0.9), ra(B–Hb) 124, ra(B–Ht) 120 pm; B–Al–B 121.5(0.7), Hb–Al–Hb 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–nMen(n= 0–3).