PARALLEL OR REVERSIBLE-REACTIONS - COMPLICATIONS IN THE SUBSTITUTION KINETICS OF OCTAHEDRAL METAL-CARBONYL-COMPLEXES

The substitution of CO by trimethyl phosphite in Mo(phen)(CO)4 and Cr(phen)(CO)4 was studied at temperatures from 25 to 55 °C and ligand concentrations of 6 X 10-3 to ~4 M in various solvents under conditions of nitrogen and carbon monoxide saturation. The reverse reaction, viz. the substitution of P(OMe)3 by CO in M(phen)(CO)3(P(OMe)3), was also investigated. In the case of molybdenum a two-term rate law with kobs = ki + kL[phosphite] was confirmed (ki = (1.79 ± 0.02) X 10~4 s-1, kL = (1.48 ± 0.05) X 10-4 M-1 s-1 at 50 °C in dichloroethane), while for chromium the rate decreased rapidly with increasing phosphite concentration under carbon monoxide saturation, reaching a limiting value of kobs = (4.37 ± 0.07) X 10-4 s-1 in dichloroethane at 50 °C. The rate of the reverse reaction for the latter case under the same conditions is (2.4 ± 0.4) X 10-3 s-1. The significance of the reverse reaction is emphasized. The two-term rate law is discussed in terms of parallel associative and dissociative pathways, all involving reversible reactions. © 1990, American Chemical Society. All rights reserved.