Tolman electronic parameter

The Tolman electronic parameter (TEP) is a measure of the electron donating or withdrawing ability of a ligand. It is traditionally determined by measuring the frequency of the A1 C-O vibrational mode (ν(CO)) of a (pseudo)-C3v symmetric complex, [LNi(CO)3] by infrared spectroscopy, where L is the ligand of interest. [LNi(CO)3] was chosen as the model compound because such complexes are readily prepared from tetracarbonylnickel(0). Analogous tetrahedral and square planar complexes, such as rhodium carbonyl chlorides, have also been utilized in measuring the chelating strength of a ligand. The shift in ν(CO) reflects how L alters metal→CO backbonding through its σ‑donor and π‑acceptor (or π‑donor) character. Strong σ‑donor/π‑acceptor ligands increase metal→CO backbonding, weakening the C≡O bond and lowering ν(CO), whereas weaker donors or π‑donors decrease backbonding and raise ν(CO). This balance between donation and back‑donation governs ligand effects on metal–ligand bond strengths, geometries, and reactivity in other complexes, providing a method of categorizing ligands in order. The analysis was introduced by Chadwick A. Tolman.