A MOLECULAR MECHANICS MODEL OF LIGAND EFFECTS .1. BINDING OF PHOSPHITES TO CR(CO)5

Molecular mechanics methods have been employed to compute the minimum energy structures of a series of eight phosphites, including trimethyl phosphite, triethyl phosphite, and triphenyl phosphite, and of their complexes with chromium pentacarbonyl, Cr(CO)5. The MMP2 force field was employed, using the comprehensive computational program BIOGRAF. Substantial changes in lowest energy conformations are seen in the complexes as compared with the free ligands. A major goal of the work was to evaluate the steric contribution to the metal-ligand interaction. As the ligand steric requirement increases, increased van der Waals repulsions between the ligand and the metal carbonyl fragment are largely offset by corresponding increases in van der Waals attractive forces. As a result, neither the overall energy nor net van der Waals interaction energies vary monotonically with increasing ligand steric requirement. The repulsive interactions between the ligand and the metal carbonyl fragment are manifested in bond angle and bond distance changes in the series.