Can you break the oxo-wall? A multiconfigurational perspective

The concept of the "oxo-wall" was conceived about 60 years ago by Harry B. Gray, and has been found to be related to the non-existence of high-valent M-oxo species in the +IV oxidation state in a tetragonal geometry beyond group 8 in the periodic table. Several efforts have been made in the past decades to test and find examples that violate this theory. Several claims of violation in the past were attributed to the difference in the geometries/coordination number and, therefore, these are not examples of true violation. In recent years, substantial efforts have been undertaken to synthesise a true Co-IV = O O species with various ligand architectures. Co-IV = O and Co-III-O-center dot are electromers and, while they are interchangeably used in the literature; the former violates the oxo-wall while the latter does not. The possibility that these two species could exist in various proportions similar to resonating structures has not been considered in detail in this area. Furthermore, there have been no attempts to quantify such mixing. In this direction, we have employed density functional theory (DFT) and ab initio CASSCF/NEVPT2 methods to quantify the co-existence of Co-IV=O and Co-III=O-center dot isomeric species. By thoroughly studying six different metal-oxo species, we affirm that the nature of such electromeric mixing is minimal/negligible for Fe-IV=O and Mn-IV=O species - both are pre-oxo-wall examples. By studying four different ligand architectures with Co-oxo species, our results unveil that the mixing of Co-IV=O <-> Co-III-O-center dot is substantial in all geometries, with dominant Co-IV=O species favourable for the S = 3/2 intermediate spin state. The percentage of the Co-III=O center dot species is enhanced substantially for the S = 1/2 low-spin state. We have attempted to develop a tool to estimate the percentage of the Co-III-O center dot species using various structural parameters. Among those tested, a linear relationship between % of Co-III-O center dot and a bond length based ratio is found (R-d = d((Co-O))/d((Co-Nax)), where d((Co-O)) and d((Co-Nax)) are the axial Co-O and Co-Na-x bond lengths in degrees A, respectively). It is found that the higher the R-d, the greater the Co-III-O-center dot character will be and the geometrically portable correlation developed offers a way to qualitatively compute the % of Co-III-O center dot character for unknown geometries.