The Tolman electronic parameter (TEP) is a measure of the electron donating or withdrawing ability of a ligand. It is traditionally determined by measuring the frequency of the A1 C-O vibrational mode (ý(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). Analogous tetrahedral and square planar complexes, such as rhodium carbonyl chlorides, have also been utilized in measuring the chelating strength of a ligand. The shift in ý(CO) reflects how L alters metalâÂÂCO backbonding through its ÃÂâÂÂdonor and ÃÂâÂÂacceptor (or ÃÂâÂÂdonor) character. Strong ÃÂâÂÂdonor/ÃÂâÂÂacceptor ligands increase metalâÂÂCO backbonding, weakening the Câ¡O bond and lowering ý(CO), whereas weaker donors or ÃÂâÂÂdonors decrease backbonding and raise ý(CO). This balance between donation and backâÂÂdonation governs ligand effects on metalâÂÂligand bond strengths, geometries, and reactivity in other complexes, providing a method of categorizing ligands in order. The analysis was introduced by Chadwick A. Tolman.
Tolman's work was preceded by previous definitions of the Metal-Ligand bond, as defined by DewarâÂÂChattâÂÂDuncanson as a combination of sigma-donation from the ligand to the metal and pi-bond "back-bonding" from the metal to the vacant ligand orbitals. Tolman himself was contemporaries with Walter Strohmeier, who along with Tolman investigated the sigma-donor ability and pi-acceptor strength of various ligands when coordinated to different metal centers. Tolman focused specifically on phosphine ligands, first cataloguing their general reactivity and then in 1970 measuring their CO frequencies seen when said ligands displace a carbon monoxide: the 70 ligands studied in his 1970 paper was the first iteration in which these vibrational frequencies were used as a parameter to determine characteristics of a ligand. Further work on phosphenes in the context of Ni(0) complexes were done: the term itself was coined in 1977, when Tolman utilized these bonding frequencies to describe the net donor properties of several phosphine ligands Since then, the scope of what is measurable through the Tolman Electronic Parameter has expanded greatly, and several resources are available for near-exhaustive lists of ligands' strengths measured through this method.
In Ni(CO)<sub>3</sub>L complexes, the three CO ligands are arranged in a pseudoâÂÂC<sub>3v</sub> geometry. Group theory shows that the three CâÂÂO stretching vibrations transform as 2Aâ + E: of these, only one A<sub>1</sub> mode corresponds to the totally symmetric, inâÂÂphase stretching of all three CO ligands. This A<sub>1</sub> mode is both IRâÂÂactive and relatively isolated in the spectrum, giving a single, wellâÂÂresolved ý<sub>(CO)</sub> band that can be assigned unambiguously.
Due to the geometry of the complex, these vibrations are symmetric. Its frequency, therefore is especially sensitive to changes in the overall electron density at the metal center. Upon coordination of CO to a metal, ý(CO) typically decreases from 2143 cm<sup>âÂÂ1</sup> of free CO. Stronger ÃÂâÂÂdonor / ÃÂâÂÂdonor ligands increase àbackbonding, weakening all three Câ¡O bonds further and lowering the A<sub>1</sub> ý<sub>(CO)</sub>, whereas weaker donors or ÃÂâÂÂacceptor ligands decrease backbonding and hence shows a smaller decrease in ý<sub>(CO)</sub>. Monitoring only this A<sub>1</sub> symmetric stretch therefore provides a clean, reproducible probe of the net electronic influence of L, which is the basis of the Tolman electronic parameter.
Tolman's original 1977 paper exclusively featured phosphines, utilizing tri-tert-butylphosphine as a baseline given its extremely basic nature. Because TEP is really a general measure of how a ligand alters metalâÂÂCO backbonding, though, the same idea can be applied to ligand classes beyond phosphines, such as NâÂÂheterocyclic carbenes (NHCs), and even to different metalâÂÂcarbonyl reference complexes. Further work done by Arduengo in the field of carbenes led to some of these N-heterocyclic carbene (NHC) ligands to be ranked according to IR spectral data recorded on cis-[RhCl(NHC)(CO)<sub>2</sub>] complexes.
A large limiting factor to the TEP is the fact that a clear, quantitative trend directly between the electronic parameter and the metal-ligand bond strength is absent. Additionally, its inability to separate ÃÂâÂÂdonor from ÃÂâÂÂacceptor contributions, exclusivity to mono-dentate ligands, and its sensitivity to experimental conditions, have raised needs for a revised electronic parameter.
Several schemes in literature use Tolman's work to use other metal centers to rank the donor properties of ligands. Work by Robert Crabtree introduced a method to test chelating phosphines, with more current work showing good correlation to previously existing literature / TEP. The HEP scale ranks ligands on the basis of the <sup>13</sup>C NMR shift of a reference ligand. A. B. P. Lever's electronic parameter ranking utilizes the Ru(II/III) couple. A competing scale utilized Chromium metal centers instead, evaluating ligands on the basis of the redox couples of [Cr(CO)<sub>5</sub>L]<sup>0/+</sup>. Hammett Substituent Constants, given that they measure the electronic influence of different substances relative to a baseline, can also be considered a useful parameter to compare against TEP when relevant. The toxicity of the precursor to the TEP, Nickel tetracarbonyl, as well as some ligands of interest not coordinating well to the Nickel center, has led to research towards finding alternatives to TEP.