Metal Olefin Complexes: Revisiting the Dewar-Chatt-Duncanson Model and Deriving Reactivity Patterns from Carbon-13 NMR Chemical Shift

Metal olefin complexes that are ubiquitous intermediates in catalysis are investigated by a detailed analysis of their C-13-NMR chemical shift tensors. This analysis allows evidencing specific electronic features, namely the olefin-to-metal sigma-donation and the metal-to-olefin pi-backdonation as proposed in the Dewar-Chatt-Duncanson model. Apart from these interactions, the chemical shift tensor analysis reveals an additional ligand-to-metal pi-donation of the olefin sigma(C=C) orbital in systems with suitably oriented vacant d-orbitals. This interaction which is not accounted for in the Dewar-Chatt-Duncanson model explains the reactivity of this type of metal olefin complexes towards oxidative cyclization (olefin insertion) and protonolysis.