The theory of "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 +4 oxidation state with tetragonal C4v symmetry beyond group 8 elements. 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 [MIV/V(15-TMC)(O)(CH3CN)]m+ (M= Mn, Fe and Co) species. Our study reveals several surprising outcomes: (i) while existing literature typically indicates the presence of either CoIV=O or CoIII-O• species beyond the wall, we propose a quantum mechanical mixture of these two species, with the dominance of one dictated by ligand design and symmetry considerations
(ii) we observe that the oxyl radical character increases beyond the wall, correlating with larger Ntrans-M-O tilt angles
and (iii) we identify an inverse relationship between the percentage of M-O• radical character and the kinetic barriers for C-H bond activation. These findings challenge traditional perspectives on the roles of oxidation states, spin states and the nature of the metal ion in reactivity.