Physical organic chemistry of transition metal carbene complexes. 15. Kinetic and thermodynamic acidities of (methylthiomethoxycarbene)pentacarbonyl complexes of chromium and tungsten in aqueous acetonitrile

Rate constants for the reversible deprotonation of (CO)(5)Cr=C(SCH3)CH3 (9-Cr) and (CO)(5)W=C(SCH3)CH3 (9-W) by OH-, water, a number of primary aliphatic and secondary alicyclic amines, and a series of carboxylate ions have been determined in 50% MeCN-50% water at 25 degrees C. This represents the first such study of Fischer carbene complexes stabilized by a thioalkoxy group. Comparison of our results to those of the corresponding methoxy derivatives (CO)5Cr=C(OCH3)CH3 (2-Cr) and (CO)(5)W=C(OCH3)CH3 (2-W) reveals that the change in the pi-donor group from MeO to MeS leads to (1) a strong increase in the thermodynamic acidity, (2) an increase in the dependence of the pK(a) on the nature of the metal, (3) a decrease in the intrinsic rate constants for proton transfer, and (4) a decrease in the dependence of the intrinsic rate constant on the metal. Observations 1, 2, and 4 can be attributed to the weaker pi-donor capability of the MeS group while observation 3 is the combined result of a steric effect due to the larger size of the MeS group and of the weaker electron-withdrawing inductive/field effect of the MeS group.