MOLECULAR MECHANICS (MM2) CALCULATIONS AND CONE ANGLES OF PHOSPHINE-LIGANDS

Molecular mechanics (MM2) calculations were performed on 54 conformati ons of 18 phosphines (PH3; PH3-nRn, where n = 1,...3, and R = Me and E t, n = 1 or 2 and R =(i)Pr, and n = 1 and R = (t)Bu, PMe2Et, PMeEt2, P PhMe2, and PPh2R where R = Me, Et, (i)Pr, (t)Bu and Ph). The results a re compared to those previously obtained from MINDO/3 and MNDO calcula tions, and to experimental data. Single conformer cone angles and weig hted average cone angles were calculated from MM2 optimized geometries employing Tolman's general definition, and they are compared to Tolma n's values, MINDO/3 results, and T.L. Brown's E(R) values. Of the cone angle definitions used, the weighted average values are suggested as the best single representation of phosphine ligand sizes. The steric p arameters (cone angle and E(R) values) alone, and in conjunction with electronic parameters, are correlated with experimental data.