A MOLECULAR MECHANICS MODEL OF LIGAND EFFECTS .4. BINDING OF AMINES TO CR(CO)5 - ER VALUES FOR AMINES

Molecular mechanics methods have been applied to compute the energy-mi nimized structures of a series of 27 alkylamines and their complexes w ith Cr(CO)5. The MMP2 force field was employed, using the comprehensiv e computational program BIOGRAF. The computed structures of Cr(CO)5(L) (L = piperidine or quinuclidine) are in good agreement with the struc tural parameters determined by X-ray diffraction studies. In general, substantial changes in the lowest energy conformations are observed in the complexes as compared with the free ligands. The energy-minimized structures are employed to calculate a new measure of ligand steric e ffect, the ligand repulsive energy, E(R), obtained by computing the va n der Waals repulsive force acting between the ligand and Cr(CO)5 frag ments along the Cr-N axis, at the equilibrium Cr-N distance, r(e). The repulsive force is scaled by r(e) to obtain E(R): E(R) = r(e)[dE(vdw) (repulsive)/dr(Cr-N)]. The amine E(R) values correlate well with cone angle for amines (r(corr) = 0.94). The amine E(R) values are compared with those of phosphines. The advantages of E(R) as a measure of ligan d steric effect are also described.