The paper describes a method for predicting the “C-O stretching” frequencies of mononuclear transition metal carbonyl compounds containing CO and other ligands (e.g., PF3, MeNC, N2, etc.). The method is based on theC-0 factored force field. Stretching force constants are evaluated by using the empirical relationship kCO = kd + (formula omitted) where kd is the stretching force constant for an isolated M(CO) unit with the appropriate number of d electrons and EL are “ligand effect constants”,which quantify the effect on kd of addinga ligand, L, at an angle θ to the M(CO) unit. Values of t-i are tabulated for a large number of ligands in octahedral, tetrahedral, and trigonal-bipyramidal geometries. Interaction force constants are evaluated by using the second empirical relationship k12 = A-B[(k1- k2)/2] where k12 is the interaction constant between CO groups 1 and 2, and A and B are empirical constants, which depend on the angle between the two groups. An additional parameter is necessary in both equations when they are applied to ionic species. The method usually predicts C-O stretching frequencies with an error of ±4 cm-1 and is easily extendable to ligands not specifically covered in this paper. © 1979, American Chemical Society. All rights reserved.
