Physical Organic Chemistry of Transition Metal Carbene Complexes. 15.¹ 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(SCH 3 )CH 3 ( 9 -Cr) and (CO) 5 W C(SCH 3 )CH 3 ( 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 °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) 5 Cr C(OCH 3 )CH 3 ( 2 -Cr) and (CO) 5 W C(OCH 3 )CH 3 ( 2 -W) reveals that the change in the π-donor group from MeO to MeS leads to (1) a strong increase in the thermodynamic acidity, (2) an increase in the dependence of the p K 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 π-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.