Metal-Oxyl Species and Their Possible Roles in Chemical Oxidations

Metal-oxyl (M-O^•) complexes having an oxyl radical ligand, which are electronically equivalent to well-known metal-oxo (M⁽═O) complexes, are surveyed as a new category of metal-based oxidants. Detection and characterization of M-O^• species have been made in some cases, although proposals and characterization of the species are mostly done on the basis of density functional theory (DFT) calculations. The reactivity of M-O^• complexes will provide a way to achieve potentially difficult oxidative conversion of substrates. This Viewpoint will provide state-of-the-art knowledge on the M-O^• species in terms of the formation, characterization, and DFT-based proposals to shed light on the characteristics of the intriguing oxidatively active species.