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 pseudotetrahedral geometries in particular, to stabilize what would otherwise be highly reactive species. However, the bonding structure in d(6) 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 d(6) Co-III oxo and imide complexes. These analyses suggest that a triple-bond description is inaccurate because of a dramatically weakened sigma 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 that of other formally doubly bonded complexes.

Automated summary

Late-transition-metal oxo and imide complexes are important in catalytic functionalization and activation of small molecules. The use of lower coordination numbers and pseudotetrahedral geometries has been seen as a way to stabilize otherwise highly reactive species, but the bonding structure in d(6) complexes in this geometry remains ambiguous. Recent experimental evidence suggests significant empirical differences between these complexes and other triply bonded compounds.