26,9. These differences are easy to understand. The former process involves the Lewis acidity of the specific site and the stability of the resulting CIP. In contrast, [Me2Al] + abstraction leads directly to an OSIP and so it is the weakly coordinating nature of the anion that is most important here. Insertion barriers correlate with the latter aspect since the transition structure 5 is also an OSIP, though one should bear in mind that those barriers are relative to CIP 2 and so the stability of the latter (relative to reactive neutrals) is also important, and which we believe contributes to the high energy barriers in the case of sheets 2a-d. We do note that the intermediate π complex 4 is also an OSIP and that it is often lower in E than the corresponding CIP + monomer (Table ). In the case of 2e the π complex is actually marginally lower in ΔG-qh-tr as noted elsewhere in the case of [SBIZrMe(π-C3H6)][B(C6F5)4]. Perhaps this is a pre-requisite for highly active catalysts in that the binding process is largely entropic in nature. Finally, we have demonstrated that two components of MAO might cooperate to form a weakly coordinating anion, and that a process of this sort could account for the exceptionally high activities seen at high Al:Zr ratios in solution for MAO-activated metallocene catalysts. The change in ion-pairing that is known to occur during metallocene catalyst activation, as evidenced by UV-Vis, NMR and ESI-MS (as well as polymerization activity) might have many explanations and ours is just a plausible one. Future work should focus on these issues and from a theoretical perspective, understanding the dynamics of these complex molecules would seem to be essential. Al:Zr ratio as simulated using COPASI. The red data is plotted on the right-hand ordinate axis while everything else is on the left-hand axis. The dashed blue line is a fit of the logistic function to the blue simulation data points, while the red dashed line is a cubic spline fit.