Abstract
N-heterocyclic carbenes (NHCs) are probably the class of ligands that not only has attracted the most attention during the past decade, but also for which the greatest advances have been made. These include a wider availability, applicability and understanding. In this chapter, an overview of all aspects of NHCs is given, starting with an historical discussion that begins almost a century ago. An inventory of the structural diversity of NHCs found in the literature is given, followed by the nomenclature of NHCs and the trivial names used. A section is devoted to the synthetic strategies developed for the formation of NHC-precursors, NHC ligands and NHC-complexes. The most diagnostic spectroscopic features of NHCs and NHC complexes are listed as well as the manner in which NHCs are usually represented. NHCs have become indubitably one of the most important and unique class of ligands as they have very distinctive and interesting electronic and steric features. A large section of this chapter is hence devoted to the discussion of these features and presents the recent advances made for determination of NHC properties and their understanding.