Computational investigation of ethylene insertion into the metal-methyl bond of first-row transition metal(III) species

Ethylene coordination and insertion into the transition metal-methyl bond h ave been investigated using nonlocal density functional theory (DFT) for th e lowest spin states of [(eta (1),eta (5)-H2NC2H4C5H4)M(III)Me](+) (M = Sc- Co) compounds. Benchmark tests at the CASPT2 level confirm that a DET appro ach with correction of spin contamination adequately describes the potentia l surfaces for this reaction as well as the separation of the various spin states. The calculations demonstrate the importance of having a single low- lying unoccupied frontier orbital available for bond formation in the pi co mplex and the transition state (TS) region. A reactant complex with nine oc cupied valence orbitals around the metal, present for example in the high-s pin d(4) configuration, is not expected to act as an efficient olefin polym erization catalyst. An empty orbital can, however, be created by spin pairi ng, which then allows the formation of a pi complex with a covalent metal-e thylene bond. This bond must be broken during insertion, and as a result, h igh barriers for the low-spin complexes are to be expected. The calculation s are consistent with observations for existing M(III)-based olefin polymer ization catalysts. Highly active catalysts are predicted for Sc and also fo r V and Co, whereas Mn(III) complexes are not expected to show significant activity.