A SIMPLE REPRESENTATION OF STERIC BULK FOR LIGANDS BY A MODIFIED VAN-DER-WAALS ENERGY EXPRESSION

The van der Waals interaction energy, DELTAE(vdw), between PX3 and the three metal fragments Cr(CO)5, Pd(PX3)3, and Pd(PX3)2 has been studie d by a modified all-repulsive van der Waals energy term, E(vdw)(mod), within the framework of molecular mechanics. The interaction energy, D ELTAE(vdw)(mod), provides a much better correlation with the Tolman co ne angle than DELTAE(vdw)(rep/att) from the original van der Waals ene rgy expression employed by the MMP2 force field, E(vdw)(rep/att). The original E(vdw)(rep/att) term has an attractive component, and in many cases affords DELTAE(vdw)(rep/att) interaction energies that are incr easingly stabilizing as the steric bulk of PR3 grows. The replacement of E(vdw)(rep/att) by E(vdw)(mod) in the MMP2 energy expression does n ot significantly change the optimized structures. It is finally sugges ted that the total difference in the MMP2 energy between L(n)M-L' on t he one hand and L' and L(n)M on the other might provide a good measure in absolute terms for the steric ligand interaction energy between L' and L(n)M, provided that E(vdw)(rep/att) is replaced by E(vdw)(mod).