Steering ligands for catalysts: N-heterocyclic carbenes

Ever since the isolation of the first free N-heterocyclic carbene more than 20 years ago, this ligand class has become essential in modern chemistry. This development is decisively owed to the stability of the metal-carbon bond, the smooth fine-tuning of ligand properties, and the facile synthetic access to such complexes. Acting as steering ligands, they are integral parts of many organometallic compounds that currently push the frontiers of chemistry in many areas, for example the synthesis and application of immobilized, water soluble, or asymmetric systems that act as efficient catalysts in organic transformations. Additionally, metal NHC complexes find promising applications in fields beyond catalysis, such as medicine, optics, and material science.