Generalization of the Tolman electronic parameter: the metal-ligand electronic parameter and the intrinsic strength of the metal-ligand bond.

The catalytic activity of transition metal complexes (R)M-L can be predicted utilizing the metal-ligand electronic parameter (MLEP) that is based on the local stretching force constant of the M-L bond. Vibrational spectroscopy is an excellent tool to accurately determine vibrational mode properties such as stretching frequencies. These correspond, because of mode-mode coupling, to delocalized vibrational modes, which have to be first converted into local vibrational modes, and their properties. Each bond of a molecule can be uniquely characterized by the local stretching force constant and frequency. The former is ideally suited to set up a scale of bond strength orders, which identifies weak M-L bonds with promising catalytic activity. It is shown how the MLEP replaces the TEP (Tolman Electronic Parameter), which is based on the CO stretching frequencies of a (CO)M-L complex and which is now exclusively used in hundreds of investigations. However, the TEP is at best a qualitative parameter that suffers from relatively large mode-mode coupling errors and the basic deficiency of most indirect descriptors: They cannot correctly describe the intrinsic M-L bond strength via the CO stretching frequencies.