Tolman electronic parameter
Encyclopedia
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.
Chadwick A. Tolman
Chadwick A. Tolman is an American chemist. He obtained his B.S. in Chemistry from Massachusetts Institute of Technology. He earned his Ph.D. in Chemistry as a microwave spectroscopist from U.C. Berkeley under the guidance of Professor William Gwinn.He joined DuPont Central Research in 1965. ...
, 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
Infrared spectroscopy
Infrared spectroscopy is the spectroscopy that deals with the infrared region of the electromagnetic spectrum, that is light with a longer wavelength and lower frequency than visible light. It covers a range of techniques, mostly based on absorption spectroscopy. As with all spectroscopic...
, 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.
| L | ν(CO) cm−1 |
|---|---|
| P(t-Bu)3 | 2056.1 |
| PMe3 Trimethylphosphine Trimethylphosphine is the organophosphorus compound with the formula P3, commonly abbreviated PMe3. This colorless liquid has a strongly unpleasant odour, which is characteristic of alkylphosphines. It is a pyramidal molecule with C3v symmetry, similar to ammonia and phosphine . As a ligand, its... |
2064.1 |
| PPh3 Triphenylphosphine Triphenylphosphine is a common organophosphorus compound with the formula P3 - often abbreviated to PPh3 or Ph3P. It is widely used in the synthesis of organic and organometallic compounds. PPh3 exists as relatively air stable, colorless crystals at room temperature... |
2068.9 |
| P(OEt)3 Triethyl phosphite Triethylphosphite is an organophosphorus compound with the formula P3, often abbreviated P3. This colorless liquid is used as a ligand in organometallic chemistry and as a reagent in organic synthesis... |
2076.3 |
| PCl3 Phosphorus trichloride Phosphorus trichloride is a chemical compound of phosphorus and chlorine, having chemical formula PCl3. Its shape is trigonal pyramidal. It is the most important of the three phosphorus chlorides. It is an important industrial chemical, being used for the manufacture of organophosphorus compounds... |
2097.0 |
| PF3 Phosphorus trifluoride Phosphorus trifluoride , is a colorless and odorless gas. It is highly toxic and it reacts slowly with water. Its main use is as a ligand in metal complexes... |
2110.8 |
The Tolman cone angle and TEP has been used to characterize the steric and electronic properties of phosphines, which are popular ligands for catalysts.