Olefin insertion using <i>ansa</i> -zirconocenes and methylaluminoxane (MAO) involves Janus-like, sheet anions

Two isomeric contact ion-pairs <i>rac</i>-Me₂Si(η⁵-C₉H₆)₂ZrMe-μ-Me-(MeAlO)₁₆(Me₃Al)₆ (1a and 1b), differing only in the detailed structure of the sheet counter-anion, are the active species in zirconocene-catalyzed olefin polymerization, mediated by methylaluminoxane (MAO), the most industrially important activator. Theoretical studies of novel, sheet models for the most abundant anion [(MeAlO)₁₆(Me₃Al)₆Me]^- detected in mixtures of MAO and metallocene complexes show that the propene insertion barriers for 1a and 1b have large differences with the more reactive species 1a having insertion barriers in good agreement with experiments. The reactivity of these isomeric ion-pairs would vary by more than two-orders of magnitude and has important implications for polymerization catalysis.