Tolman electronic parameter
# Tolman electronic parameter

The Tolman electronic parameter (TEP) is a measure of the electron donating or withdrawing ability of a ligand. It is determined by measuring the frequency of the $A_1$ C-O vibrational mode (ν(CO)) of a $(pseudo)-C_{3v}$ 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). The shift in ν(CO) is used to infer the electronic properties of a ligand, which can aid in understanding its behavior in other complexes. The analysis was introduced by Chadwick A. Tolman.

The $A_1$ carbonyl band is rarely obscured by other bands in the analyte's infrared spectrum. Carbonyl is a small ligand so steric factors do not complicate the analysis. Upon coordination of CO to a metal, ν(CO) typically decreases from 2143 $cm^{−1}$ of free CO. This shift can be explained by π backbonding: the metal forms a π bond with the carbonyl ligand by donating electrons through its d orbitals into the empty π* anti-bonding orbitals on CO. This interaction strengthens the metal-carbon bond but also weakens the carbon-oxygen bond, resulting in a lower vibrational frequency. If other ligands increase the density of π electrons on the metal, the C-O bond is weakened and ν(CO) decreases further; conversely, if other ligands compete with CO for π backbonding, ν(CO) increases.

TEP for selected phosphines :

| L | ν(CO) $cm^{−1}$ |
| - | - |
| $P(t-Bu)_3$ | 2056.1 |
| $P(NMe_2)_3$ | 2061.9 |
| $PMe_3$ | 2064.1 |
| $P(C_6H_4OMe)_3$ | 2066 |
| $PPh_3$ | 2068.9 |
| $P(C_6H_4F)_3$ | 2071.3 |
| $P(OEt)_3$ | 2076.3 |
| $PCl_3$ | 2097.0 |
| $PF_3$ | 2110.8 |

## Other ligand electronic parameters

Several other scales have been proposed for the ranking of the donor properties of ligands. The HEP scale ranks ligands on the basis of the $^{13}C$ NMR shift of a reference ligand. Lever's electronic parameter ranking is related to the Ru(II/III) couple. Another scale evaluated ligands on the basis of the redox couples of $[Cr(CO)_5L]^{0/+}.$

In a treatment akin to the TEP analysis, the donor properties of N-heterocyclic carbene (NHC) ligands have been ranked according to IR data recorded on $cis-[RhCl(NHC)(CO)_2]$ complexes.

