A Molecular Mechanics Model of Ligand Effects. 4. Binding of Amines to Cr(CO)₅: E_R Values for Amines

Molecular mechanics methods have been applied to compute the energy-minimized structures of a series of 27 alkylamines and their complexes with Cr(CO)₅. The MMP2 force field was employed, using the comprehensive computational program BIOGRAF. The computed structures of Cr(CO)₅(L) (L = piperidine or quinuclidine) are in good agreement with the structural 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 effect, the ligand repulsive energy, E_R, obtained by computing the van der Waals repulsive force acting between the ligand and Cr(CO)₅ fragments 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 ligand steric effect are also described.