Richard S. Herrick' and paul J. Botelho
Department of Chemistry. College of the Holy Cross, Worcester, Massachusetts, 01610
Received October 12, 1991; accepted January 22, 1992
Equilibrium constants between (3.9(1.2) \times 10^{3}) and (7.3(1.7) \times 10^{4} \mathrm{M}^{-1}) are reported for the reaction of (\mathrm{Mo}(\mathrm{CO}){2}(\mathrm{DCE})\left(\mathrm{PR}{3}\right){2} \mathrm{X}{2}\left(\mathrm{X}=\mathrm{Cl}, \mathrm{Br} ; \mathrm{PR}{3}=\mathbf{P}\left(\mathrm{C}{6} \mathrm{H}{4}-p-\mathrm{OMe}\right){3}, \mathrm{PPh}{3}, \mathrm{PEt}{3}\right.); (\mathrm{DCE}=1,2)-dichloroethane) with (\mathrm{CO}) to form (\mathrm{Mo}(\mathrm{CO}){3}\left(\mathrm{PR}{3}\right){2} \mathrm{Br}{2}). The equilibrium constant for addition of (\mathrm{CO}) to the unstable species (\left(\eta^{5}-\mathrm{C}{5} \mathrm{H}{5}\right) \mathrm{Mo}(\mathrm{CO}){2}(\mathrm{DCE}) \mathrm{Cl}), obtained by comparison of the rate constant for addition of (\mathrm{CO}) to the dicarbonyl with the known rate constant for (\mathrm{CO}) dissociation from (\left(\eta^{3}-\mathrm{C}{5} \mathrm{H}{5}\right) \mathrm{Mo}(\mathrm{CO}){3} \mathrm{Cl}), is (2.5(0.9) \times 10^{9} M^{-1}). Free energy differences are calculated for these compounds and for the previously studied system, (\mathrm{Mo}(\mathrm{CO}){2}\left(o-\mathrm{C}{6} \mathrm{H}{4} \mathrm{Cl}{2}\right)\left(\mathrm{S}{2} \mathrm{CNEt}{2}\right){2}). It is concluded that transition state stabilization of (\mathrm{Mo}(\mathrm{CO}){2}(\mathrm{Solv})\left(\mathrm{PR}{3}\right){2} \mathrm{X}{2}) and of (\mathrm{Mo}\left(\mathrm{CO}{2}\left(\mathrm{Solv}{2}\right)\left(\mathrm{S}{2} \mathrm{CNEt}{2}\right){2}(\mathrm{Solv}=\right.) solvent ()), which in fact is due to the stability of the 16 -electron dicarbonyls, accounts for the observed differences in ligand substitution rates for the tricarbonyl systems. โ 1993 Academic Press, Inc.