Trimethylaluminium: Scrambling of methyl groups in the dimer and exchange with trimethylgallium

Interchange of methyl groups between the bridging and terminal positions of trimethylaluminium dimer and exchange of methyl groups between trimethyl- aluminium and trimethylgallium in cyclopentane have been re-examined, again by means of proton magnetic resonance spectroscopy. Dissociation of the dimer is confirmed as the rate- determining step in both cases, but the rate of dissociation is much greater in toluene than in cyclopentane. The evidence previously put forward for a cage effect limiting the rate of exchange with trimethylgallium in cyclopentane is refuted and a revised mechanism for exchange is suggested.