Physical organic chemistry of transition metal carbene complexes. 21. Kinetics and mechanism of hydrolysis of (CO)(5)M=C(SR)Ar (M = Cr and W; R = CH3and 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 r eported. 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 formati on 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 i ntermediate (T-OH(-)), followed by product formation via five potential pat hways 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. Thi s understanding is in large measure based on an analysis of how the intrins ic rate constants are affected by the interplay of steric, inductive, and p i -donor effects and the potential imbalances of these effects at the trans ition state. It is also shown that the much lower sensitivity to H+-catalys is of RS- compared to RO- leaving group departure from the tetrahedral inte rmediate is responsible for the more complex rate-pH profile for the hydrol ysis of (CO)(5)M=C(SMe)Ph than for the hydrolysis of the methoxy analogue.