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Zusammenfassung:
Ethylene coordination and insertion into the transition metal−methyl bond have been investigated using nonlocal density functional theory (DFT) for the lowest spin states of [(η1,η5-H2NC2H4C5H4)M(III)Me]+ (M = Sc−Co) compounds. Benchmark tests at the CASPT2 level confirm that a DFT approach with correction of spin contamination adequately describes the potential 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 π complex and the transition state (TS) region. A reactant complex with nine occupied valence orbitals around the metal, present for example in the high-spin d4 configuration, is not expected to act as an efficient olefin polymerization catalyst. An empty orbital can, however, be created by spin pairing, which then allows the formation of a π complex with a covalent metal−ethylene bond. This bond must be broken during insertion, and as a result, high barriers for the low-spin complexes are to be expected. The calculations are consistent with observations for existing M(III)-based olefin polymerization catalysts. Highly active catalysts are predicted for Sc and also for V and Co, whereas Mn(III) complexes are not expected to show significant activity.
