Stabilization of organometallic species achieved by the use of N-heterocyclic carbene (NHC) ligands

This review describes the stabilization of various coordinatively unsaturated metal centers through the incorporation of N-heterocyclic carbene (NHC) ligands. Such species are more thermally stable compared to the more ubiquitous tertiary phosphane systems. Although NHCs can be considered as phosphane mimics it has become apparent that there are substantial differences between the two ligand families. In general, NHC ligands are much more electron-donating and sterically demanding than bulky phosphane ligands. We also discuss the various thermochemical and infrared studies of metal carbene species that have provided insight into NHC ligands properties, and at last allowed meaningful comparison to tertiary phosphane ligands. While NHCs are generally viewed as strong binding low reactive ligands, in some instances they have been found to be not so innocent and can undergo facile intramolecular C-H activation as well as abnormal ligand binding [C-5(4) vs. C-2]. Here we highlight such reactions with late-transition metal centers that have allowed the isolation of various unsaturated LTM species. These have so far eluded isolation in analogous phosphane-based systems. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).