Characterizing the Metal–Ligand Bond Strength via Vibrational Spectroscopy: The Metal–Ligand Electronic Parameter (MLEP)

The field of organometallic chemistry has tremendously grown over the past decades and become an integral part of many areas of chemistry and beyond. Organometallic compounds find a wide use in synthesis, where organometallic compounds are utilized as homogeneous/heterogeneous catalysts or as stoichiometric reagents. In particular, modifying and fine-tuning organometallic catalysts has been at the focus. This requires an in-depth understanding of the complex metal–ligand (ML) interactions which are playing a key role in determining the diverse properties and rich chemistry of organometallic compounds. We introduce in this article the metal–ligand electronic parameter (MLEP), which is based on the local vibrational ML stretching force constant, fully reflecting the intrinsic strength of this bond. We discuss how local vibrational stretching force constants and other local vibrational properties can be derived from the normal vibrational modes, which are generally delocalized because of mode–mode coupling, via a conversion into local vibrational modes, first introduced by Konkoli and Cremer. The MLEP is ideally suited to set up a scale of bond strength orders, which identifies ML bonds with promising catalytic or other activities. The MLEP fully replaces the Tolman electronic parameter (TEP), an indirect measure, which is based on the normal vibrational CO stretching frequencies of [RnM(CO)mL] complexes and which has been used so far in hundreds of investigations. We show that the TEP is at best a qualitative parameter that may fail. Of course, when it was introduced by Tolman in the 1960s, one could not measure the low-frequency ML vibration directly, and our local mode concept did not exist. However, with these two problems solved, a new area of directly characterizing the ML bond has begun, which will open new avenues for enriching organometallic chemistry and beyond.