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

Abstract

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 ΔE_S–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, pK_a and nucleophilicity index values for the MNHCs were found to be in good agreement with their calculated electron donation ability.