Evaluation of the σ-Donating and π-Accepting Properties of N-Heterocyclic Boryl Anions

The relative a-donating and pi-accepting capacities of a range of synthetically relevant boryl anions have been evaluated by examining the geometric, thermo-chemical, and electronic properties of their adducts to the Li+ cation and Se atom, as compared to the properties of the analogous neutral N-heterocyclic carbenes (NHCs), by theoretical methods. The results indicate that boryl anions have a weaker pi-accepting capability compared to NHCs, but it is still a non-negligible factor in the bonding contributions between boryl and the Se atom. The tunability of the pi-accepting capacity of boryl anions is similar to that of NHCs, indicating a potential for the modification of the electronic properties of metal complexes incorporating either boryl or NHC ligands. In all cases, the boryl ligands were found to be superior sigma-donors to NHCs.