Evaluation of the Sigma Donating and Pi Accepting Properties of N-Heterocyclic Boryl Anions

The relative σ donor and π accepting capacities of a range of synthetically relevant boryl anions have been evaluated by examining the geometric, thermochemical and electronic properties of their adducts to the Li+ cation and Se atom, as compared to the analogous neutral NHCs, by theoretical methods. The results indicate that boryl anions have a weaker π accepting capability than NHCs, but it is still a nonnegligible factor in the bonding contributions between boryl and the Se atom. The tuneability of the π accepting capacity of boryl anions is similar to that of NHCs, indicating a similar potential for 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 σ donors to NHCs