In search of stable singlet metalla-N-heterocyclic carbenes (MNHCs): a contribution from theory.

Theoretical studies predict that the stability of the singlet state of metalla-N-heterocyclic carbenes (MNHCs) is strongly influenced not only by the substituents at the α-nitrogen atoms and the nature of ligands at the transition metal center but also by the substituents at the carbenic backbone. All the MNHCs were found to have a stable singlet ground state and the computed ΔES-T values for some of the MNHCs were found to be significantly large and lie within the range of experimentally known carbenes (31.0-84.0 kcal mol-1). Furthermore, they were found to have superior σ-donation ability to conventional NHCs. The calculated proton affinities, gallium pyramidalization, pKa and nucleophilicity index values for the MNHCs were found to be in good agreement with their calculated electron donation ability.