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 species with various ligand architectures. Co IV O and Co III -O˙ 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˙ 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˙ 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˙ 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˙ species using various structural parameters. Among those tested, a linear relationship between % of Co III -O˙ and a bond length based ratio is found (, where d (Co-O) and d (Co-Nax) are the axial Co-O and Co-N ax bond lengths in Å, respectively). It is found that the higher the R d , the greater the Co III -O˙ character will be and the geometrically portable correlation developed offers a way to qualitatively compute the % of Co III -O˙ character for unknown geometries.