Physical organic chemistry of transition metal carbene complexes.: 21.: Kinetics and mechanism of hydrolysis of (CO)5M=C(SR)Ar (M = Cr and W; R = CH3 and CH3CH2CH2; Ar=C6H5 and 3-ClC6H4) in aqueous acetonitrile.: Important differences relative to complexes with alkoxy leaving groups

A kinetic study of the hydrolysis of (CO)(5)M=C(SMe)Ph (M = Cr and W) in 50% MeCN-50% water (v/v) at 25 degreesC over a pH range from 1.7 to 14.2 is reported. The reaction occurs in two stages: the first is formation of (CO)(5)M=C(O-)Ph or (CO)(5)M=C(OH)Ph while the second stage leads to the formation of PhCH=O and (CO)(5)MOH-. This paper is concerned with the first stage. The rate-pH profiles are complex and consistent with a mechanism (Scheme 1) that involves water/OH- addition to the substrate to form a tetrahedral intermediate (T-OH(-)), followed by product formation via five potential pathways whose relative importance depends on the pH. A more limited study of the reactions of (CO)(5)M=C(SCH2CH2CH3)Ph (M = Cr and W) and (CO)(5)M=C(SMe)C6H4-3-Cl (M = Cr and W) with OH- is also reported. The main focus of the discussion is aimed at understanding the reactivity differences between (CO)(5)M=C(SMe)Ph and the corresponding methoxy analogues studied earlier. This understanding is in large measure based on an analysis of how the intrinsic rate constants are affected by the interplay of steric, inductive, and pi -donor effects and the potential imbalances of these effects at the transition state. It is also shown that the much lower sensitivity to H+-catalysis of RS- compared to RO- leaving group departure from the tetrahedral intermediate is responsible for the more complex rate-pH profile for the hydrolysis of (CO)(5)M=C(SMe)Ph than for the hydrolysis of the methoxy analogue.