Cooperative bond activation reactions with carbene complexes.

Metal-ligand cooperation has emerged as a powerful concept in bond activation chemistry and catalysis. Here, both the metal and the ligand are directly involved in the bond breaking and bond formation processes, while in "classical" activation reactions the transformations solely take place at the metal centre. Several different types of cooperating ligand systems have been developed over the years and applied in catalysis. In this Feature Article, we will discuss the use of carbene ligands in cooperative bond activation reactions. We focus on two types of carbene complexes: mid and late transition metal carbene species (i) with PCcarbeneP pincer ligands and (ii) with strong anion-stabilizing α-substituents at the carbene carbon atom. These systems are active in a variety of bond activation reactions and have found first catalytic applications.