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Joined 24 July 2024

Currently working on: Tolman Electronic Parameter

The A1 "stretch" mode of Ni(CO)3L used to determine the TEP of L.

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] 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.

The reaction used to determine a ligand's Tolman Electronic Parameter.

Inspiration and Discovery

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Tolman's work was preceded by previous definitions of the Metal-Ligand bond, as defined by Dewar–Chatt–Duncanson 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.[2][3] 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.[4][5] Tolman focused specifically on phosphine ligands, first cataloguing their general reactivity and then in 1970 measuring the different 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 stretch frequencies were used as a parameter to determine characteristics of a ligand.[6] Further work on phosphenes in the context of Ni(0) complexes were done: [7] the term itself was coined in 1977, when Tolman utilized these bonding frequencies to describe the net donor properties of several phosphine ligands[8] 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. [9][10]

Theory & methodology

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Phosphines

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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.

The backbonding interaction weakens the carbon-oxygen bond, resulting in a lower vibrational frequency of CO. [11]
TEP for selected phosphines[8]
Lν(CO) cm−1
P(t-Bu)32056.1
P(NMe2)32061.9
PMe32064.1
P(C6H4OMe)32066
PPh32068.9
P(C6H4F)32071.3
P(OEt)32076.3
PCl32097.0
PF32110.8

N-Heterocyclic Carbenes (NHCs)

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Tolman's original 1977 paper exclusively featured phosphines, utilizing tri-tert-butylphosphine as a baseline given its extremely basic nature. However, further work done by Arduengo in the field of carbenes[12][13] led to some of these N-heterocyclic carbene (NHC) ligands to be ranked according to IR data recorded on cis-[RhCl(NHC)(CO)2] complexes.[14][15] Studies now use TEP values as an easy experimental method to access important electronic data of NHCs. [16] Similarly, work on utilizing a TEP-esque system on an NHC-Iridium complex has been reported as well.[17]

Other ligand electronic parameters

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Several other scales have been proposed for the ranking of the donor properties of ligands. Different transition metals of Nickel, for example, have been measured with carbon monoxide ligands and hence their variation of HEP.[18][19]The HEP scale ranks ligands on the basis of the 13C NMR shift of a reference ligand.[20] A. B. P. Lever's electronic parameter ranking utilizes the Ru(II/III) couple.[21] A competing scale utilized Chromium metal centers instead, evaluating ligands on the basis of the redox couples of [Cr(CO)5L]0/+.[22] More recently, a generalized form of electronic parameters have been studied in order to directly measure the strength of a Metal-Ligand bond without the intermediary of carbon dioxide disassociation.[23][24]

See also

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References

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  1. ↑ Robert H. Crabtree (2005). "Carbonyls, Phosphine Complexes, and Ligand Substitution Reactions". The Organometallic Chemistry of the Transition Metals. pp. 87–124. doi:10.1002/0471718769.ch4. ISBN 9780471718765.
  2. ↑ Dewar, M. J. S. (1951). "A review of π Complex Theory". Bulletin de la Société Chimique de France. 18: C71.
  3. ↑ Chatt, J; Duncanson, L. A. (1953). "Olefin co-ordination compounds. Part III. Infra-red spectra and structure: Attempted preparation of acetylene complexes". Journal of the Chemical Society: 2939–2947.
  4. ↑ Strohmeier, W.; Müller, F. J. (1967). "Notizen: π-Acceptorstärke von Phosphinen als Liganden in Cyclopentadienylmangantricarbonyl und Nickelcarbonyl". Zeitschrift für Naturforschung B. 22b: 451–452.
  5. ↑ Strohmeier, W.; Müller, F. J. (1967). "Klassifizierung phosphorhaltiger Liganden in Metallcarbonyl-Derivaten nach der π-Acceptorstärke". Chemische Berichte. 100: 2812–2821.
  6. ↑ Tolman, C. A. (1970). "Electron Donor-Acceptor Properties of Phosphorus Ligands. Substituent Additivity". Journal of the American Chemical Society. 92: 2953–2956.
  7. ↑ Tolman, C. A.; Seidel, W. C.; Gosser, L. W. (1974). "Formation of three-coordinate nickel(0) complexes by phosphorus ligand dissociation from NiL₄". Journal of the American Chemical Society. 96 (1): 53–60.
  8. 1 2 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.
  9. ↑ Setiawan, D.; Kalescky, R.; Kraka, E.; Cremer, D. (2016). "[Title of the article, if available; omit if unknown]". Inorganic Chemistry. 55: 2332–2344.
  10. ↑ Setiawan, Daniel; Kalescky, Rebecca; Kraka, Erika; Cremer, Dieter (2017). "Ligand ranking in transition metal complexes: direct assessment of ligand electronic effects by using vibrational spectroscopy and quantum chemical calculations". Physical Chemistry Chemical Physics. 19 (8): 5707–5722. doi:10.1039/C6CP07793E.
  11. ↑ Hoffmann, R. (1963). "An Extended Hückel Theory. I. Hydrocarbons". The Journal of Chemical Physics. 39 (6): 1397–1412. Bibcode:1963JChPh..39.1397H. doi:10.1063/1.1734456.
  12. ↑ 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. PMID 27829895.
  13. ↑ 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. PMID 29601194.
  14. ↑ Arduengo, Anthony J. III; Harlow, Richard L.; Kline, Michael (1991). "A Stable Crystalline Carbene". Journal of the American Chemical Society. 113: 361–363.
  15. ↑ Arduengo, Anthony J. III; Dias, Raul; Harlow, Richard L; Kline, Michael (1992). "Electronic and structural congruence of stable, crystalline carbene (NHC) ligands and their complexes". Journal of the American Chemical Society. 114: 5530–5534.
  16. ↑ Rayner, P. J.; Norcott, P.; Appleby, K. M. (2018). "Fine-tuning the efficiency of para-hydrogen-induced hyperpolarization by rational N-heterocyclic carbene design". Nature Communications. 9: 4251. doi:10.1038/s41467-018-06766-1.
  17. ↑ Kelly, R. A. III; Clavier, H.; Giudice, S.; Scott, N. M.; Stevens, E. D.; Bordner, J.; Samardjiev, I.; Hoff, Jr., C. D.; Cavallo, L.; Nolan, S. P. (2008). "Determination of N-heterocyclic carbene (NHC) Steric and Electronic Parameters Using the [(NHC)Ir(CO)2Cl] System". Organometallics. 27: 202–210.
  18. ↑ Kühl, Olaf (2005). "Predicting the net donating ability of phosphines—do we need sophisticated theoretical methods?". Coordination Chemistry Reviews. 249 (5–6): 693–704. doi:10.1016/j.ccr.2004.08.021. ISSN 0010-8545.
  19. ↑ Kalescky, R.; Kraka, E.; Cremer, D. (2014). "New Approach to Tolman's Electronic Parameter Based on Local Vibrational Modes". Inorganic Chemistry. 53: 478–495. doi:10.1021/ic4024663.
  20. ↑ Teng, Qiaoqiao; Huynh, Han Vinh (2017). "A Unified Ligand Electronic Parameter Based on C NMR Spectroscopy of N-Heterocyclic Carbene Complexes". Dalton Transactions. 46 (3): 614–627. doi:10.1039/C6DT04222H. PMID 27924321.
  21. ↑ Lever, A. B. P. (1990). "Electrochemical parametrization of metal complex redox potentials, using the ruthenium(III)/Ruthenium(II) couple to generate a ligand electrochemical series". Inorganic Chemistry. 29 (6): 1271–1285. doi:10.1021/ic00331a030.
  22. ↑ Chatt, Joseph; Kan, C. T.; Leigh, G. Jeffery; Pickett, Christopher J.; Stanley, David R. (1980). "Transition-metal binding sites and ligand parameters". Journal of the Chemical Society, Dalton Transactions (10): 2032. doi:10.1039/DT9800002032.
  23. ↑ Cremer, Dieter; Kraka, Elfi (2017). "Generalization of the Tolman electronic parameter: the metal–ligand electronic parameter and the intrinsic strength of the metal–ligand bond". Dalton Transactions. 46: 8323–8343. doi:10.1039/C7DT00178A.
  24. ↑ Setiawan, D.; Kraka, E.; Cremer, D. (2015). "Direct Measure of Metal–Ligand Bonding and Its Impact on the Electronic Character of Transition Metal Complexes: A Local Vibrational Mode Study". The Journal of Physical Chemistry A. 119: 9541–9556. doi:10.1021/acs.jpca.5b05863.

Further reading

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