Kinetics and Mechanism of Alkaline Hydrolysis of [(Methoxy)(p-substituted styryl)-carbene] Pentacarbonyl Chromium(0) Complexes in Aqueous Acetonitrile

Abstract

Kinetic studies have been performed for alkaline hydrolysis of a series of [(methoxy)(p-substituted styryl)carbene]pentacarbonyl chromium(0) complexes (<TEX>$(CO)_5$</TEX>Cr=<TEX>$C(OCH_3)CH=CHC_6H_4X$</TEX>, X = p-<TEX>$OCH_3$</TEX>, p-<TEX>$CH_3$</TEX>, H, p-Cl, p-<TEX>$NO_2$</TEX>). Second-order rate constants <TEX>$(k_{{OH}^-})$</TEX> for the alkaline hydrolysis in 50% acetonitrile-water(v/v) were determined spectrophotometrically at various temperatures. At a low pH region (pH < 7.5), the observed rate constant <TEX>$(k_{obs})$</TEX> remained constant with a small value, while in a high pH region (pH > 9.5), <TEX>$k_{obs}$</TEX> increases linearly with increasing the pH of the medium. The second-order rate constants <TEX>$(k_{{OH}^-})$</TEX> increase as the substituent X changes from a strong electron donating group to a strong electron withdrawing group. The Hammett plot obtained for the alkaline hydrolysis is consisted of two intersecting straight lines. The nonlinear Hammett plot might be interpreted as a change in the rate-determining step. However, the fact that the corresponding Yukawa-Tsuno plot is linear with <TEX>$\rho$</TEX> and r values of 0.71 and 1.14, respectively indicates that the nonlinear Hammett plot is not due to a change in the rate-determing step but is due to ground-state stabilization through resonance interaction. The positive <TEX>$\rho$</TEX> value suggests that nucleophilic attack by <TEX>$OH^-$</TEX> to form a tetrahedral addition intermediate is the rate-determining step. The large negative <TEX>${\Delta}S^\neq$</TEX> value determined in the present system is consistent with the proposed mechanism.