Oligomeric structures of the crystalline dimethylamine adducts Me-2(H)N center dot MH3 (M = Al or Ga) and the dimethylamido derivative [Me2NGaH2](3)

The adduct Me-2(H)N . AlH3 (1) has been isolated for the first time at low temperatures from the reaction of [Me2NH2]Cl with LiAlH4 in Et2O solution at ca. 250 K. Single crystals of this and of the corresponding gallane Me-2(H)N . GaH3 (2) have been grown, also at low temperature; the structures of these at 150 K determined by X-ray diffraction reveal dimeric units Me-2(H)N .H2M(mu -H)(2)MH2.N(H)Me-2 with highly unsymmetrical M-H . . .M hydrogen bridges, as well as evidence of significant intermolecular N-H . . .H-M interactions, particularly for M=Al. The M-N distances are quite similar [1 2.028(1), 2 2.079(3) Angstrom], but the M . . .M distance in 2 [3.7519(15) Angstrom] is much longer than in 1 [2.8743(8) Angstrom]. The adducts decompose at ambient temperatures, rapidly in the case of 1, slowly in the case of 2, with the elimination of H-2 and the formation of the corresponding dimethylamido derivative. Unlike the vapour at low pressure in which the dimeric molecule predominates, crystals of the gallane (3) at 150 K are composed of trimers [Me2NGaH2](3) structurally analogous to the corresponding alane, with a chair-like conformation and a mean Ga-N distance of 1.981(3) Angstrom.