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.