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The Tolman electronic parameter (TEP), named after Chadwick A. Tolman, is a measure of the electron donating or withdrawing ability of a ligand. It is determined by measuring the frequency of the A1 C-O vibrational mode of a complex, LNi(CO)3 by infrared spectroscopy, where L is the ligand being studied. LNi(CO)3 was chosen as the model compound because such complexes are readily prepared from tetracarbonylnickel(0).
The carbonyl band is quite distinctive, and 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 to a metal, ν(CO) typically decreases from 2143 cm−1 of free CO. This can be explained by π backbonding: the metal is able to form a π bond with the carbonyl ligand by donating electrons through its d orbitals into the empty π* anti-bonding orbitals on CO. This strengthens the metal-carbon bond, but also weakens the carbon-oxygen bond. If other ligands increase the density of π electrons on the metal, the C-O bond is weakened and ν(CO) decreases; conversely, if other ligands compete with CO for π backbonding, ν(CO) increases.
The Tolman cone angle and TEP has been used to characterize the steric and electronic properties of phosphines, which are popular ligands for catalysts.