Tolman electronic parameter

It is 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).

[1][2] 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.

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.

[5] 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.