Abnormal, mesoionic and remote N-heterocyclic carbene complexes

Three different structural classes of NHC ligands can be distinguished: normal (nNHC), abnormal (aNHC), alternatively called mesoionic (MIC), and remote (rNHC). General principles, synthetic strategies as well as recent results in the area of transition metal complexes of aNHC/MICs and rNHCs are discussed. The special properties of aNHC/MICs are discussed including their somewhat debateable status as true carbenes in the full sense of the term, as contrasted with their close analogy with nNHCs in the bound state. Some applications to catalysis are included and future prospects outlined. (C) 2012 Elsevier B.V. All rights reserved.