BONDING AND LIGAND-EXCHANGE IN PENTAMETHYLDIALANE - AN IR, NMR AND AB-INITIO STUDY

IR studies have shown that the major constitutent of a labile gas-phas e equilibrium in mixtures of trimethylalane and dimethylalane is the h eteroleptic pentamethyldialane, Me(2)Al(mu H)(mu Me)AlMe(2). A partial normal coordinate analysis of the spectra of isotopic variants reveal s that this predominance arises from a significant strengthening of th e Al-H-Al bridging bond. The experimental observations are supported b y ab initio calculations of model systems. IR and variable temperature H-1 NMR studies lead to similar conclusions for solutions of alane mi xtures in toluene, and lend support for a ligand-exchange mechanism in volving singly bridged species.