Direct experimental evaluation of ligand-induced backbonding in nickel metallacyclic complexes

The details of ligand-induced backbonding in nickel diphosphine pi complexes are explored using nickel L-edge (3d <- 2p) X-ray absorption spectroscopy as a means of quantifying the degree of backbonding derived from direct Ni 3d donation into the pi ligand. It is observed that backbonding into weakly pi acidic ligands such as alkenes and arenes is dominated by contributions from the diphosphine ligand via sigma-donation, leading to activated metallacycles with a Ni(0) d(10) metal centre. With more strongly pi acidic ligands, however, metal contributions to backbonding increase substantially leading to a more electron-deficient metal centre that is best described as having a Ni(i) spectroscopic oxidation state.