High-Valent Late Transition Metal-Oxo Complexes: Breaking Boundaries at the Oxo Wall and Beyond.

High-valent first-row transition metal-oxo species are active intermediates in oxidation reactions crucial to many biological processes, oxidative chemical transformations in industry, and water splitting for renewable energy generation. The direct activation and hydroxylation of unactivated aliphatic C-H bonds remain a challenging yet important transformation in efficiently synthesizing complex organic molecules and pharmaceuticals with oxygenated functionalities. Despite documenting over 100 synthetic Fe-O species, terminal metal-oxo complexes involving the late transition metals Co, Ni, and Cu are much rarer, primarily due to the "oxo wall" phenomenon. The additional electrons in the metal-oxo unit weaken the M-O bond and destabilize these species. At the same time, as one traverses past the oxo wall, the M-O unit gains increasing amounts of metal-oxyl character, which holds promise for the generation of highly active catalysts for C-H bond activation, as exemplified by copper monooxygenase enzymes in nature. While forming late transition metal-oxo complexes is challenging, a few have been successfully prepared by targeting specific symmetries or unusual spin states. This review explores nonheme iron oxo complexes, the "oxo wall" concept, and recent advances in overcoming this limitation to prepare metal-oxo complexes of cobalt, nickel, and copper beyond the oxo wall.