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-phase equilibrium in mixtures of trimethylalane and dimethylalane is the heteroleptic pentamethyldialane, Me2Al(µH)(µMe)AlMe2. A partial normal coordinate analysis of the spectra of isotopic variants reveals that this predominance arises from a significant strengthening of the Al—H—Al bridging bond. The experimental observations are supported by ab initio calculations of model systems. IR and variable temperature 1H 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.