Rate and equilibrium constants for the nucleophilic attachment of various thiolate ions to (CO)(5)M= C(SMe)Ph (M = Cr and W) and for the attachment of HOCH2CH2S- to (CO)(5)W=C(SCH2CH2OH)C(6)H(4)Z (Z = CF3, Cl, F, H, Me, MeO, and Me2N) have been determined in 50% MeCN-50% water (v/v) at 25 degrees C. For some of the adducts, those of the general structure (CO)(5)MC(SR,SR')C(6)H(4)Z, a kinetic determination of the pK(a) of their metal-protonated conjugate acid, (CO)(5)M(H)C(SR,SR')C(6)H(4)Z, is also reported. On the basis of Br phi nsted coefficients, Hammett rho values and comparisons with the reactions of the same thiolate ions with methoxy carbene complexes such as (CO)(5)M=C(OMe)Ph, insights into the role played by desolvation of the nucleophile prior to C-S bond formation, by the smaller pi-donor and inductive but larger steric effects of the MeS compared to the MeO group, by transition state imbalances, etc. are discussed. The pK(a) values obtained for (CO)(5)M(H)C(SR,SR')C(6)H(4)Z were used to resolve a mechanistic ambiguity in the hydrolysis of Fischer carbenes that have acidic protons on the alpha-carbon, e.g., (CO)(5)Cr=C(OMe)Me.