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.