Two, three, or not to be? Elucidating Multiple Bonding in d6 Pseudotetrahedral Oxo and Imide Complexes

Late transition metal oxo and imide complexes play an important role in the catalytic functionalization and activation of small molecules. An emerging theme in this area over the past few decades has been the use of lower-coordination numbers, and pseudo-tetrahedral geometries in particular, to stabilize what would otherwise be highly reactive species. However, the bonding structure in d⁶ oxo and imide complexes in this geometry is ambiguous. These species are typically depicted with a triple bond, however recent experimental evidence suggests significant empirical differences between these complexes and other triply bonded complexes with lower d-counts. Here we use a suite of computational orbital localization methods and electron density analyses to probe the bonding structure of isoelectronic d⁶ Co(III) oxo and imide complexes. These analyses suggest that a triple bond description is inaccurate due to a dramatically weakened σ interaction. While the exact bond order in these cases is necessarily dependent on the model used, several metrics suggest that the strength of the metal–O/N bond is most similar to other formally doubly bonded complexes.