(3) using the cluster results. In cycle (1), where the carbon chain length doubles and 1-butene is formed (species 1 -5 ), the ethylene insertion involves an activation energy (1.76 eV) higher than the olefin liberation step (1.02 eV). Indeed, previous studies have shown that ethylene insertion is rate-determining in this system, and the cluster calculations predict that the energy of a type I TS is typically higher than a type II. Throughout the energy landscape of cycle (2), three type I elementary steps have distinctly high activation energies: the ethylene insertion shared by cycle