The molecular geometries of AI(BH,)Me, and Ga(BH,)Me, in the gas phase have been studied by electron diffraction. Both species appear to be monomeric molecules with skeletons having C , , symmetry and containing bidentate tetrahydroborate groups. Salient parameters t for AI(BH,)Me, are : r,(AI-C) 192.9(0.4), r,,(Al-B) 21 2.8(0.8), r,(AI-H,,) 177.0(3.2), ra(B-Hb) 123.0(1.7), ra(B-Ht) 120.7(1.7) pm; C-AI-C 1 1 8.4(0.7), Hb-AI-Hb 70.6(4.8)' (t = terminal, b = bridging). The corresponding parameters t for Ga(BH,)Me, are: r,(Ga-C) 194.4(0.4), r,(Ga-B) 21 6.3(0.8), ra(Ga-Hb) 179.1 (3.0), I,( B-H,,) 121.7(1.9), ra( B-Hi) 1 1 9.2(1.9) pm; C-Ga-C 1 1 8.8(1.2), H,-Ga-H,, 68.4(4.6)". Exchange of methyl for tetrahydroborate groups in molecules of the type M(BH,),-,(CH,), ( M = Al or Ga, n = 2 or 3; M = Al, n = 0) evokes but small changes in the dimensions of the (CH,),M and M(BH,) groups although there are signs suggestive of modest variations of the bonding within these two groups.250 4 72 85 500 2 32 45 1 000 1 13 22 Ga(BH,)Me, 250 4 64 120 500 2 30 45 1 000 1 17 20 borate (m.p. 1-2 "C) each judged to be pure by the criteria (i) of its vapour pressure and (ii) of the i.r. spectrum of its vapour.'. 16* l7 Our experiments have shown that gaseous 15-45 "C #GaMe, + SGaCl, -+ Li[BH,] GaMe,Cl -Ga(BH,)Me, (2) -15 "C