What Is the Real Catalyst in M/NHC Systems? From Molecular Complexes to Cocktail-Type Catalysis

Complexes of transition metals with N-heterocyclic carbine (NHC) ligands are often introduced and optimized as well-defined molecular precatalysts. However, the collected mechanistic evidence shows that M/NHC systems do not operate in a single universal manner. Depending on the metal, substrate, base, temperature, and ligand environment, the same precatalyst may function in a classical molecular mode with a preserved M–NHC bond, or it may evolve into an NHCdisconnected catalytic manifold compose