Bridging the Oxo Wall: A New Perspective on High‐valent Metal‐Oxo Species and Their Reactivity in Mn, Fe, and Co Complexes

Abstract The “oxo‐wall” is a well‐established concept in the area of bioinorganic chemistry, which refers to the instability of the terminal metal‐oxo complexes in the +4 oxidation state, with tetragonal C 4v symmetry beyond group 8 elements. This leads to a diverse and highly reactive chemistry of Co‐oxo complexes, as evidenced in the literature, ranging from challenging C−H bond activation to efficient water oxidation. Despite extensive research on first‐row terminal metal‐oxo complexes and the “oxo‐wall” concept, studies correlating the reactivity of these species across the periodic table remain scarce. In this work, using a combination of DFT and ab initio CASSCF calculations, we have explored the structure, bonding, and reactivity of [M IV/V (15‐TMC)(O)(CH 3 CN)] m+ (M= Mn, Fe and Co) species. Our study reveals several intriguing outcomes: (i) while existing literature typically indicates the presence of either Co IV =O or Co III –O • species beyond the wall, we propose a quantum mechanical mixture of these two species (termed as Co IV =O Co III –O • ), with the per cent of mixing dictated by ligand architecture and symmetry considerations; (ii) we observe that the oxyl radical character increases beyond the wall, correlating with larger N trans ‐M−O tilt angles; and (iii) we identify an inverse relationship between the percentage of M–O • character and the kinetic barriers for C−H bond activation. These findings offer a new perspective on the roles of oxidation states, spin states, and the nature of the metal ion in reactivity.