Group 3 tetrahydroborates. Part 2. The molecular structures of dimethylaluminium tetrahydroborate and dimethylgallium tetrahydroborate in the gas phase as determined by electron diffraction

Abstract

The molecular geometries of Al(BH₄)Me₂ and Ga(BH₄)Me₂ in the gas phase have been studied by electron diffraction. Both species appear to be monomeric molecules with skeletons having C_2v, symmetry and containing bidentate tetrahydroborate groups. Salient parameters for Al(BH₄)Me₂ are: r_a(Al–C) 192.9(0.4), r_a(Al–B) 212.8(0.8), r_a(Al–H_b) 177.0(3.2),r_a(B–H_b) 123.0(1.7), r_a(B–H_t) 120.7(1.7) pm; C–Al–C 118.4(0.7), H_b–Al–H_b 70.6(4.8)°(t = terminal, b = bridging). The corresponding parameters for Ga(BH₄)Me₂ are: r_a(Ga–C) 194.4(0.4), r_a(Ga–B) 216.3(0.8), r_a(Ga–H_b) 179.1 (3.0), r_a(B–H_b) 121.7(1.9), r_a(B–H_t) 119.2(1.9) pm; C–Ga–C 118.8(1.2), H_b–Ga–H_b 68.4(4.6)°. Exchange of methyl for tetrahydroborate groups in molecules of the type M(BH₄)_3–n(CH₃)_n(M = Al or Ga, n= 2 or 3; M = Al, n= 0) evokes but small changes in the dimensions of the (CH₃)₂M and M(BH₄) groups although there are signs suggestive of modest variations of the bonding within these two groups.