Physical organic chemistry of transition metal carbene complexes.: 16.: Reactions of (CO)5M=C(OR)Ph (M = Cr or W; R = Me or Et) with thiolate ions in aqueous acetonitrile.: Complete kinetic dissection of the two-step mechanism

A kinetic study of the reactions of (CO)(5)M=C(OR')Ph (1a, M = Cr, R' = Me; 1b, M = W, R' = Me; 1c, M = Cr, R' = Et; 1d, M = W, R' = Et) with n-PrS-, HOCH2CH2S-, MeO2CCH2CH2S-, and MeO2CCH2S- in 50% MeCN-50% water (v/v) at 25 degrees C is reported. At low [RS-] and relatively low pH the reaction leads to the nucleophilic substitution products (CO)(5)M=C(SR)Ph without accumulation of any intermediate. At high [RS-] and high pH formation of a tetrahedral intermediate, (CO)(5)(M) over bar C(OR')(SR)Ph, is observed. Upon addition of acid the intermediate is converted into the substitution product. For the reactions of most thiolate ions a detailed kinetic analysis allowed the determination of the rate constants for nucleophilic attack on the carbene complexes (k(1)) and its reverse (k(-1)), the equilibrium constant for nucleophilic addition (K-1 = k(1)/k(-1)), and the rate constants for alkoxide ion departure from the respective intermediates catalyzed by H+ (k(2)(H)) and N-methylmorpholinium ion (k(2)(BH)). The dependence of these rate and equilibrium constants on the metal, the leaving group, and the thiolate ion and comparisons with reactions of thiolate ions with other electrophiles provide insights into the transition state structure, the resonance effect of the (CO)(5)M moieties, the requirements for partial desolvation of the nucleophile prior to entering the transition state, and transition state imbalances.