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.

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► The carbenoid status of the title carbenes in the free vs. bound state is discussed.
► aNHCs have a higher donor power and trans influence than nNHCs.
► aNHCs are finding increasing use in homogeneous catalysis.