N-Heterocyclic carbenes: state of the art in transition-metal-complex synthesis

Recently, transition-metal complexes containing nucleophilic N-heterocyclic carbenes (NHCs) have been receiving increased attention in the literature, largely due to their favorable application in homogeneous catalysis. This article reviews the principles of known preparative procedures of such complexes that have been developed over the past 30 years since the independent isolation of chromium and mercury imidazolin-2-ylidene complexes by Öfele and Wanzlick. The discussion will focus exclusively on the preparation of complexes of nitrogen heterocycles and makes no claim to be exhaustive. These complexes exhibit all the principles of NHC ligands and appear most frequently in the literature.