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

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)(CH₃CN)]^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.