A MOLECULAR MECHANICS MODEL OF LIGAND EFFECTS .3. A NEW MEASURE OF LIGAND STERIC EFFECTS

Molecular mechanics methods have been employed to compute energy-minimized structures for a large number of Cr(CO)5L complexes, where L is CO, trialkyl- or triarylphosphine, mixed trialkyl/arylphosphine, phosphite, alkyl/aryl/alkoxyphosphine, or one of 8 analogous arsenic ligands. The energy-minimized structures are employed to compute a new measure of ligand steric effect, the ligand repulsive energy, E(R), obtained by computing the van der Waals repulsive force acting between the ligand and Cr(CO)5 fragments along the Cr-P axis, at the equilibrium Cr-P distance, r(e). The repulsive force is multiplied by r(e) to obtain E(R): E(R) = [partial derivative E(vdw)(repulsive)/partial derivative r(Cr-P)]r(e). Values of E(R) correlate very well with Tolman's cone angle values, theta, for all phosphines. Phosphites and arsenic ligands depart from the same correlation, but the overall correlation for 69 ligands is fair (r(corr) = 0.857). The comparative effectiveness of E(R) and theta in linear free energy correlations of kinetics and equilibrium data is analyzed for a large body of literature results. E(R) and theta are comparably effective in terms of a superficial analysis based on correlation coefficients alone. The limitations and advantages of E(R) as a measure of ligand steric effect are described.