IR studies have shown that the major constitutent of a labile gas-phase equilibrium in mixtures of trimethylalane and dimethylalane is the heteroleptic pentamethyldialane, Me,Al(pH)(pMe)AIMe, . A partial normal coordinate analysis of t h e spectra of isotopic variants reveals that this predominance arises from a significant strengthening of t h e AI-H-AI bridging bond. The experimental observations are supported by ab initio calculations of model systems. IR and variable temperature ' H NMR studies lead to similar conclusions for solutions of alane mixtures in toluene, and lend support for a ligand-exchange mechanism involving singly bridged species.Interest in the chemistry of simple alkyl derivatives of alane and gallane has recently undergone something of a renaissance, One factor influential in this revival is the widespread utilisation of these compounds as precursors to the deposition of A1 and Ga in semiconductor materials via processes such as metal organic chemical vapour deposition (MOCVD).' Thus both trimethylalane (TMA), Me,Al, and trimethylgallane, Me,Ga, have been used as Group 13 sources in the compound semiconductors GaAs and AlGaAs., However, these methyl compounds have not proved entirely satisfactory, largely because of substantial contamination by carbon, particularly in the case of Al., This fact has initiated a search for viable and economic alternative precursors. In the case of Ga, the problem has largely been solved by the use of triethylgallane, Et,Ga, where the lower activation /3-hydride elimination route effectively prevents production of the alkyl radicals widely held to be the principal culprits in carbon c ~n t a m i n a t i o n . ~~~ Unfortunately, the corresponding triethylalane is rendered a less practical source by its very low vapour pressure. This has led to searches in other directions, one of which has identified dimethylalane (DMA), Me,AlH, as a possible candidate for MOCVD pur-Paper 5/02063H ;
