N -Heterocyclic Carbenes as Ligands in Metal Complexes

This article provides a survey on N -heterocyclic carbenes (NHCs) as ligands to transition metal complexes. Even though NHCs are a relatively young class of ligands, especially when compared with classical donor systems such as amines, imines, phosphines, and sulfides, these ligands have become very rapidly highly popular and constitute now a standard tool in organometallic and coordination chemistry and in the application of metals for a variety of purposes. This article provides a brief historical context, outlines the key properties of NHCs as ligands in terms of electronic and steric behavior, and introduces the different methodological concepts to install the metal center on this type of ligands in a conceptual rather than a comprehensive manner. In the second part, applications are briefly highlighted, including a short survey on catalytic hallmarks in particular and also other domains such as biomedical and electronic applications. Finally, this article concludes with emphasizing some limitations, misconceptions, and some future perspectives in this fascinating area of chemistry.