NMR evidence for B(C6F5)3 attack on the inward position of a highly hindered meso ansa-zirconocene

The reaction of the ansa-meso-[C2H4(4,7-Me2Indenyl)2]ZrMe2 with B(C6F5)3 afforded, besides the expected ion-pair with outward MeB(C6F5)3- (a), a minor isomer (b, 4.3%) with the bulky methylborate anion in the inward site. Evidence of migration of MeB(C6F5)3- between the two sites and of B(C6F5)3 attack on the (inward) methyl site of a has been obtained.