Activation of the C–N Bond of N-heterocyclic Carbenes by Inorganic Elements

N-heterocyclic carbenes (NHCs) are generally considered to be extremely robust, ideal spectator ligands. However, in the past two years there have been several reports of the activation of the endocyclic C–N bond with subsequent insertion into element–hydrogen and element–carbon bonds. The new reaction pathway casts the reactivity of the NHC heterocyclic ring into a new light, especially in the context of potential catalyst decomposition. The synthetic reports and theoretical studies in this rapidly developing area are summarized here