Physical Organic Chemistry of Transition Metal Carbene Complexes. 17.¹ Kinetics of the Reactions of (Arylthioalkoxycarbene)pentacarbonyl Complexes of Chromium(0) and Tungsten(0) with Thiolate Ions in Aqueous Acetonitrile:  pKa Values of the Metal-Protonated Tetrahedral Adducts Formed between Carbene Complexes and Thiolate Ion

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 HOCH 2 CH 2 S - to (CO) 5 W C(SCH 2 CH 2 OH)C 6 H 4 Z (Z = CF 3, Cl, F, H, Me, MeO, and Me 2 N) have been determined in 50% MeCN−50% water (v/v) at 25 °C. For some of the adducts, those of the general structure (CO) 5 M̄C(SR,SR‘)C 6 H 4 Z, a kinetic determination of the p K 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 Brønsted coefficients, Hammett ρ 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 π-donor and inductive but larger steric effects of the MeS compared to the MeO group, by transition state imbalances, etc. are discussed. The p K 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 α-carbon, e.g., (CO) 5 Cr C(OMe)Me.