Ligand cone angle
In coordination chemistry, the ligand cone angle (
Asymmetric cases
[edit]The concept of cone angle is most easily visualized with symmetrical ligands, e.g. PR3. But the approach has been refined to include less symmetrical ligands of the type PRR′R″ as well as diphosphines. In such asymmetric cases, the substituent angles' half angles,
Ligand | Angle (°) |
---|---|
PH3 | 87[1] |
PF3 | 104[1] |
P(OCH3)3 | 107[1] |
dmpe | 107 |
depe | 115 |
P(CH3)3 | 118[1] |
dppm | 121 |
dppe | 125 |
dppp | 127 |
P(CH2CH3)3 | 132[1] |
dcpe | 142 |
P(C6H5)3 | 145[1] |
P(cyclo-C6H11)3 | 179[1] |
P(t-Bu)3 | 182[1] |
P(C6F5)3 | 184[1] |
P(C6H4-2-CH3)3 | 194[1] |
P(2,4,6-Me3C6H2)3 | 212 |
Variations
[edit]The Tolman cone angle method assumes empirical bond data and defines the perimeter as the maximum possible circumscription of an idealized free-spinning substituent. The metal-ligand bond length in the Tolman model was determined empirically from crystal structures of tetrahedral nickel complexes. In contrast, the solid-angle concept derives both bond length and the perimeter from empirical solid state crystal structures.[5][6] There are advantages to each system.
If the geometry of a ligand is known, either through crystallography or computations, an exact cone angle (
Application
[edit]The concept of cone angle is of practical importance in homogeneous catalysis because the size of the ligand affects the reactivity of the attached metal center. In an example,[10] the selectivity of hydroformylation catalysts is strongly influenced by the size of the coligands. Despite being monovalent, some phosphines are large enough to occupy more than half of the coordination sphere of a metal center. Recent research has found that other descriptors—such as percent buried volume—are more accurate than cone angle at capturing the relevant steric effects of the phosphine ligand(s) when bound to the metal center.[11]
See also
[edit]- Steric effects (versus electronic effects)
- Tolman electronic parameter
References
[edit]- ^ a b c d e f g h i j k Tolman, Chadwick A. (1970-05-01). "Phosphorus ligand exchange equilibriums on zerovalent nickel. Dominant role for steric effects". J. Am. Chem. Soc. 92 (10): 2956–2965. doi:10.1021/ja00713a007.
- ^ Tolman, C. A.; Seidel, W. C.; Gosser, L. W. (1974-01-01). "Formation of three-coordinate nickel(0) complexes by phosphorus ligand dissociation from NiL4". J. Am. Chem. Soc. 96 (1): 53–60. doi:10.1021/ja00808a009.
- ^ Tolman, C. A. (1977). "Steric Effects of Phosphorus Ligands in Organometallic Chemistry and Homogeneous Catalysis". Chem. Rev. 77 (3): 313–48. doi:10.1021/cr60307a002.
- ^ Manz, T. A.; Phomphrai, K.; Medvedev, G.; Krishnamurthy, B. B.; Sharma, S.; Haq, J.; Novstrup, K. A.; Thomson, K. T.; Delgass, W. N.; Caruthers, J. M.; Abu-Omar, M. M. (2007). "Structure−Activity Correlation in Titanium Single-Site Olefin Polymerization Catalysts Containing Mixed Cyclopentadienyl/Aryloxide Ligation". J. Am. Chem. Soc. 129 (13): 3776–3777. doi:10.1021/ja0640849. PMID 17348648.
- ^ Immirzi, A.; Musco, A. (1977). "A method to measure the size of phosphorus ligands in coordination complexes". Inorg. Chim. Acta. 25: L41–L42. doi:10.1016/S0020-1693(00)95635-4.[dead link]
- ^ Niksch, Tobias; Görls, Helmar; Weigand, Wolfgang (2009). "The Extension of the Solid-Angle Concept to Bidentate Ligands". Eur. J. Inorg. Chem. 2010 (1): 95–105. doi:10.1002/ejic.200900825.
- ^ Bilbrey, Jenna A.; Kazez, Arianna H.; Locklin, J.; Allen, Wesley D. (2013). "Exact ligand cone angles". Journal of Computational Chemistry. 34 (14): 1189–1197. doi:10.1002/jcc.23217. PMID 23408559. S2CID 23864226.
- ^ "AaronTools". aarontools.readthedocs.io. Retrieved 2023-05-30.
- ^ Petitjean, Michel (2015). "Analytical Algorithms for Ligand Cone Angles Calculations. Application to Triphenylphosphine Palladium Complexes". Comptes Rendus Chimie. 18 (6): 678–684. doi:10.1016/j.crci.2015.04.004.
- ^ Evans, D.; Osborn, J. A.; Wilkinson, G. (1968). "Hydroformylation of Alkenes by Use of Rhodium Complex Catalyst". Journal of the Chemical Society. 33 (21): 3133–3142. doi:10.1039/J19680003133.
- ^ Newman-Stonebraker, Samuel H.; Smith, Sleight R.; Borowski, Julia E.; Peters, Ellyn; Gensch, Tobias; Johnson, Heather C.; Sigman, Matthew S.; Doyle, Abigail G. (2021). "Univariate classification of phosphine ligation state and reactivity in cross-coupling catalysis". Science. 374 (6565): 301–308. Bibcode:2021Sci...374..301N. doi:10.1126/science.abj4213. PMID 34648340. S2CID 238991361.