Synthetic Strategies Toward NHO-Metal Complexes and Applications in Synthesis and Catalysis

N-heterocyclic olefin (NHO)-metal complex has emerged as a new class of coordination complexes. The structure of an NHO is characterized by an N-heterocyclic ring with an exocyclic C═C bond, and since its terminal carbon carries a negative charge, it is nucleophilic and basic in nature. The nucleophilic terminal carbon can bind to the metals; this is important when exploring the reactivity of the NHOs with various metal complexes. In this connection, the earlier work of Kuhn and coworkers introduced a tetramethyl-substituted NHO and corresponding metal complexes. Inspired by this work, many research groups have successfully synthesized metal complexes of the structurally different NHO (common methyl-substituted backbone, backbone-modified, N-modified, terminal carbon-substituted, etc.) ligands. Most NHO-metal complexes have been demonstrated as catalysts in various reactions including amination, coupling reactions, annulation followed by olefination, hydroboration, ring-closing metathesis (RCM), and ring-opening polymerization (ROP), and so on. Based on the current progress in the NHO-metal complexes, more developments could be anticipated. In this review, we discuss (i) the synthetic strategies used to synthesize NHO-metal complexes of the s-, p-, and d-block elements, and (ii) their recently published applications in various organic transformations and polymerizations.