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

Molecular mechanics (MM2) calculations were performed on 54 conformations of 18 phosphines (PH3; PH3-nRn, where n = 1,...3, and R = Me and Et, n = 1 or 2 and R =(i)Pr, and n = 1 and R = (t)Bu, PMe2Et, PMeEt2, PPhMe2, and PPh2R where R = Me, Et, (i)Pr, (t)Bu and Ph). The results are compared to those previously obtained from MINDO/3 and MNDO calculations, and to experimental data. Single conformer cone angles and weighted average cone angles were calculated from MM2 optimized geometries employing Tolman's general definition, and they are compared to Tolman'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 parameters (cone angle and E(R) values) alone, and in conjunction with electronic parameters, are correlated with experimental data.