The Tolman electronic parameter, often abbreviated as \( \tau \), is a measure used in coordination chemistry to describe the trans influence of ligands in a coordination complex, particularly with regard to how they affect the electronic properties of metal centers. Specifically, it provides insight into the ability of a ligand to stabilize certain oxidation states of a metal ion and how they interact electronically with the metal. The parameter is named after Richard C.
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The Tolman electronic parameter compares ligand electronic influence using the symmetric CO stretching band of a matched metal-carbonyl probe, classically . Lower frequency commonly indicates greater net electron release into the remaining CO ligands. It combines donation and acceptance and is not a direct measurement of octahedral splitting.