Physical organic chemistry of transition metal carbene complexes. 17. Kinetics of the reactions of (arylthioalkoxycarbene)pentacarbonyl complexes of chromium(0) and tungsten(0) with thiolate ions in aqueous acetonitrile: pK(a) values of the metal-protonated tetrahedral adducts formed between carbene complexes and thiolate ion

Article abstract of DOI:10.1021/ja992741+

Rate and equilibrium constants for the nucleophilic attachment of various thiolate ions to (CO)₅M=C(SMe)Ph (M=Cr and W) and for the attachment of HOCH₂CH₂S^- to (CO)₅W=C(SCH₂CH₂OH)C₆H₄Z (Z=CF₃, Cl, F, H, Me, MeO, and Me₂N) have been determined in 50% MeCN-50% water (v/v) at 25 °C. For some of the adducts, those of the general structure (CO)₅M?C(SR,SR')C₆H₄Z, a kinetic determination of the pK(a) of their metal-protonated conjugate acid, (CO)₅M(H)C(SR,SR')C₆H₄Z, is also reported. On the basis of Bronsted coefficients, Hammett ρ values and comparisons with the reactions of the same thiolate ions with methoxy carbene complexes such as (CO)₅M=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 pK(a) values obtained for (CO)₅M(H)C(SR,SR')C₆H₄Z were used to resolve a mechanistic ambiguity in the hydrolysis of Fischer carbenes that have acidic protons on the α-carbon, e.g., (CO)₅Cr=C(OMe)Me.