LIGAND STERIC PROPERTIES

Methods of estimating the steric requirements of ligands are reviewed. The most widely employed measure is the cone angle, THETA, first proposed by Tolman. Elaborations on the cone angle concept include mathematical methods for its estimation, estimates based on X-ray structural data, and solid cone angle measures. The ligand repulsive parameter, E(R), is based upon molecular mechanics calculations of the structures of Cr(CO)5 complexes of the various ligands. The THETA and E(R) parameters correlate reasonably well, but significant disparities are found among a large group of phosphorus, arsenic and nitrogen ligands for which both THETA and E(R) values are available. Consideration of the various approaches to estimating ligand steric requirements indicates that each ligand has a range of steric requirements relative to other ligands, depending on the details of the particular complex or reaction involved. Applications of ligand steric requirements include quantitative linear free energy relationships. Given the relative imprecision with which the steric (and probably also the electronic) parameters can be determined, the use of additional parameters to account for the relative importances of sigma and pi bonding, or the existence of a steric threshold may not be justified by the number and variety of data available. Nevertheless, despite their lack of high precision, linear free energy relationships can provide important information regarding the electronic and steric demands of the transition state relative to the ground state in chemical reactions.