CAACs as efficient ancillary ligands for the synthesis of robust catalysts

• CAACs form strong bonds with transition metals as well as main group elements. • CAAC ligand families (CAAC-5, CAAC-6, BiCAACs, FunCAACs, CAArCs, CAAmCs, ChiCAACs) are described. • CAAC-based transition metal complexes are shown to be robust catalysts. • Complexes with CAAC ligands find applications in coordination chemistry, catalysis, bio-inorganic chemistry, medicinal, and material science. CAACs have emerged as an attractive class of ancillary ligands, forming strong bonds with transition metals as well as main-group elements and stabilise both low and high oxidation states. These strongly nucleophilic and electrophilic carbenes have a small HOMO−LUMO energy gap compared to N -heterocyclic carbenes (NHCs). The robust metal complexes formed with CAAC ligands have been utilised in different areas such as catalysis, small-molecule activation, medicinal chemistry, and bioinorganic chemistry. This review describes recent developments in the synthesis of various types of CAAC ligands, their binding ability to transition metals and applications of CAAC−based metal complexes in homogeneous catalysis via hydrogenation, hydroboration, coupling reactions, olefin metathesis and asymmetric transformations.