Tolman electronic parameter

<p>The Tolman electronic parameter (TEP) is a measure of the electron donating or withdrawing ability of a <a href="/facts/Ligand/6etB3qeW">ligand</a>. It is determined by measuring the frequency of the A1 C-O vibrational mode (ν(CO)) of a (pseudo)-C3v <a href="/facts/Molecular_symmetry/BrDxAkTK">symmetric</a> complex, [LNi(CO)3] by <a href="/facts/Infrared_spectroscopy/u1Hzx617">infrared spectroscopy</a>, where L is the ligand of interest. [LNi(CO)3] was chosen as the model compound because such complexes are readily prepared from <a href="/facts/Tetracarbonylnickel(0)/8Py5Dre1">tetracarbonylnickel(0)</a>. 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 <a href="/facts/Chadwick_A._Tolman/Epd5S1Gn">Chadwick A. Tolman</a>.
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<p>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 <a href="/facts/Pi_backbonding/un6buomN">π backbonding</a>: 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.
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TEP for selected phosphines<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a><table><tbody><tr><th>L</th><th>ν(CO) cm−1</th></tr><tr><td>P(t-Bu)3</td><td>2056.1</td></tr><tr><td><a href="/facts/Tris(dimethylamino)phosphine/pG1MKSP9">P(NMe2)3</a></td><td>2061.9</td></tr><tr><td><a href="/facts/Trimethylphosphine/m4WG9nYz">PMe3</a></td><td>2064.1</td></tr><tr><td><a href="/facts/Tris(4-methoxyphenyl)phosphine/qSDuF1d0">P(C6H4OMe)3</a></td><td>2066</td></tr><tr><td><a href="/facts/Triphenylphosphine/nYFNCVcJ">PPh3</a></td><td>2068.9</td></tr><tr><td>P(C6H4F)3</td><td>2071.3</td></tr><tr><td><a href="/facts/Triethyl_phosphite/hQQzIEXh">P(OEt)3</a></td><td>2076.3</td></tr><tr><td><a href="/facts/Phosphorus_trichloride/PsQ1HXcl">PCl3</a></td><td>2097.0</td></tr><tr><td><a href="/facts/Phosphorus_trifluoride/feeXrTsB">PF3</a></td><td>2110.8</td></tr></tbody></table>

Tolman electronic parameter open-in-new

Tolman electronic parameter