Valence Bond Insights into the Bonding Nature of N-Heterocyclic Carbenes

The bonding characteristics of N-heterocyclic carbenes (NHCs) have long been the subject of debate, particularly regarding the relative importance of carbenic versus ylidic resonance contributions. In this work, we employ ab initio valence bond (VB) theory to systematically examine the electronic structure of a series of five-membered NHCs and closely related species. Our VB analyses reveal that carbenic and ylidic characters contribute comparably to the overall wave function, with ylidic character often more pronounced in NHCs, although their relative contributions depend sensitively on the atomic composition of the heterocyclic ring. The π framework of NHCs, arising from six electrons distributed over five pπ orbitals, exhibits substantial cyclic delocalization. In addition, our VB analysis provides insights into the bidirectional electron-delocalization processes between the carbene carbon and the adjacent atoms, namely π donation to the carbene carbon and σ donation from the carbene carbon, together with their associated energetic contributions. Taken together, these results offer a unified, quantitatively grounded description of NHC bonding, highlighting the distinctive roles of π and σ interactions in shaping the electronic structure of these chemically versatile systems.