Cone angles:: Tolman's and Plato's

The successful application of Tolman cone angles for P-donor and other ligands in accounting quantitatively for steric effects in a wide variety of physicochemical processes is contrasted with the variability of cone angles obtained from crystallographic studies. It is maintained that the latter are not relevant in describing steric effects for reactions in solution. Problems with cone angles for ligands with conformational uncertainties are best dealt with by systematic measurement of deviations of data for those ligands from trends defined by ligands with less ambiguous cone angles. In fact a body of cone angles for all ligands could be obtained by adjusting cone angles to give perfect fits to individual steric profiles and then averaging the values obtained from a large number of such studies. In this way a set of cone angles could be obtained that are divorced from their origins in Tolman's models and justified solely by their successful quantitative application. Although it is tempting to relate these cone angles to some Platonically perfect set, consideration given here to even a small number of sets of data of this sort suggests that there may always be some ligands with uncertain cone angles. Thus, a combination of eight sets of data that include P(OMe)(3) as a ligand suggests that it should have its Tolman cone angle increased by ca. 10degrees. A much larger proposed increase is disputed. Another set of data suggests that its Tolman cone angle is appropriate when it is a nucleophile but that P(O-i-Pr)(3) as a micleophile should have its Tolman cone angle reduced by ca. 5degrees. Thus the deviant behaviour of conformationally ambiguous ligands may be systematic, and could depend on whether the P-donors are acting as ligands or nucleophiles. However, the concept of a Platonically perfect set of cone angles is probably justified for many ligands that are conformationally unambiguous and these ideal cone angles may be essentially identical with Tolman's values. In the course of these analyses a new set of pi-acidity parameters was developed and values are tabulated. (C) 2002 Elsevier Science B.V. All rights reserved.