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 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. The analysis was introduced by Chadwick A. Tolman.
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3L.png.webp)
The A1 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.
| L | ν(CO) cm−1 |
|---|---|
| P(t-Bu)3 | 2056.1 |
| P(NMe2)3 | 2061.9 |
| PMe3 | 2064.1 |
| P(C6H4OMe)3 | 2066 |
| PPh3 | 2068.9 |
| P(C6H4F)3 | 2071.3 |
| P(OEt)3 | 2076.3 |
| PCl3 | 2097.0 |
| PF3 | 2110.8 |
The Tolman cone angle and TEP have been used to characterize the steric and electronic properties of phosphines, which are popular ligands for catalysts.
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.[3][4]
See also
References
- Robert H. Crabtree (2005). Carbonyls, Phosphine Complexes, and Ligand Substitution Reactions. pp. 87–124. doi:10.1002/0471718769.ch4.
- Tolman, C. A. (1977). "Steric effects of phosphorus ligands in organometallic chemistry and homogeneous catalysis". Chem. Rev. 77 (3): 313–348. doi:10.1021/cr60307a002.
- Nonnenmacher, Michael; Buck, Dominik M; Kunz, Doris (23 August 2016). "Experimental and theoretical investigations on the high-electron donor character of pyrido-annelated N-heterocyclic carbenes". Beilstein Journal of Organic Chemistry. 12: 1884–1896. doi:10.3762/bjoc.12.178. PMC 5082490.
- Huynh, Han Vinh (30 March 2018). "Electronic Properties of N-Heterocyclic Carbenes and Their Experimental Determination". Chemical Reviews. 118 (19): 9457–9492. doi:10.1021/acs.chemrev.8b00067.
Further reading
- Tonner, Ralf; Frenking, Gernot (2009). "Tolman's Electronic Parameters for Divalent Carbon(0) Compounds". Organometallics. 28 (13): 3901–3905. doi:10.1021/om900206w.
- Gusev, Dmitry G. (2009). "Electronic and Steric Parameters of 76 N-Heterocyclic Carbenes in Ni(CO)3(NHC)". Organometallics. 28 (22): 6458–6461. doi:10.1021/om900654g.
