Chemical bonding in transition metal carbene complexes

In this work, we summarize recent theoretical studies of our groups in which modern quantum chemical methods are used to gain insight into the nature of metal-ligand interactions in Fischer- and Schrock-type carbene complexes. It is shown that with the help of charge- and energy-partitioning techniques it is possible to build a bridge between heuristic bonding models and the physical mechanism which leads to a chemical bond. Questions about the bonding situation which in the past were often controversially discussed because of vaguely defined concepts may be addressed in terms of well defined quantum chemical expressions. The results of the partitioning analyses show that Fischer and Schrock carbenes exhibit different bonding situations, which are clearly revealed by the calculated data. The contributions of the electrostatic and the orbital interaction are estimated and the strength of the σ donor and π acceptor bonding in Fischer complexes are discussed. We also discuss the bonding situation in complexes with N,N-heterocyclic carbene ligands.