Reactivities of N-heterocyclic carbenes at metal centers

Since the pioneering work by Jack Hine in 1950,¹ in which he proposed carbon dichloride (i.e. a “carbene”) as a key intermediate in the hydrolysis of chloroform, carbenes have risen from being chemical curiosities to established transient intermediates and stable organic and organometallic compounds. Carbene chemistry is now a vast field, with researchers across many disciplines adding to the ever-expanding variety of fascinating structures and compounds. A particularly prolific area of carbene chemistry involves N-heterocyclic carbenes (NHCs). NHCs are nitrogen-containing heterocycles that contain a divalent carbon atom with six valence electrons. Due to the strongly sigma donating nature of the divalent carbon atom, NHCs are often deployed as ancillary ligands in organometallic chemistry. Although they are often referred to as spectator ligands, the proximity of the metal center may result in parts of the ligand architecture being involved in reactivity, leading to the formation of new and sometimes unexpected species. This chapter provides an account of the fascinating reactivity of NHCs at metal centers.