Periodic Trends in the Binding of a Phosphine-Tethered Ketone Ligand to Fe, Co, Ni, and Cu.

π-Coordinating ligands are commonly found in intermediate structures in homogeneous catalysis, and are gaining interest as supporting ligands for the development of cooperative catalysts. Herein, we systematically investigate the binding of the ketone group, a strongly accepting π ligand, to mid-to-late metals of the first transition series. To this end, the coordination of 2,2'-bis(diphenylphosphino)benzophenone (Ph dpbp), which features a ketone moiety flanked by two strongly binding P-donor groups, to Fe, Co, Ni, and Cu was explored. The ketone moiety does not bind to the metal in MII complexes, whereas MI complexes (Fe, Co, Ni) adopt an η2 (C,O) coordination. A structural and computational investigation of periodic trends in this series was performed. These data suggest that the coordination of the ketone to MI can mostly be described by the resonance extremes of the Dewar-Chatt-Duncanson model, that is, the π complex and the metallaoxacycle extreme, with a possible minor contribution from a ketyl radical resonance structure in the case of the iron complex.