Normal and abnormal N-heterocylic carbene ligands: Similarities and differences of different mesoionic C-donor complexes

Similarities and differences of normal versus abnormal carbene complexes are highlighted, with an emphasis on effects that can be directly attributed to the different bonding mode and which are thus not imparted by potential stereoelectronic effects such as more or less shielding of the MC bond. While generally, the bonding scheme is similar throughout all classes of carbenes, distinctly different behaviors have been noted in some reactivity patterns, in particular when comparing different bonding modes of imidazolylidenes. This distinction is blurred with triazolylidenes, and the normal/abnormal nomenclature is not particularly meaningful in these cases. With pyridylidenes, structural differences are noted in the ground state, while reactivity patterns are not significantly dependent on the metal binding site. Common denominators for all normal and abnormal heterocyclic carbene complexes include (i) a strongly mesoionic character of both normal and abnormal bonding modes of the ligand, and (ii) a negligibly small carbenic character. Accordingly 'carbene' is a rather debatable name for this class of ligands, and 'mesoionic C-donor' may be more accurate.