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

The relative σ-donating 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 properties of the analogous neutral N-heterocyclic carbenes
(NHCs), by theoretical methods. The results indicate that boryl anions
have a weaker π-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 π-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 σ-donors to NHCs.