Reactions of [aryloxy(phenyl)carbene]pentacarbonylchromium(0) complexes with thiolate ions. Decreasing reactivity with increasing basicity of the nucleophile

A kinetic study of the reactions of thiolate ions with three Fischer-type [aryloxy(phenyl)carbene]pentacarbonyl chromium(0) complexes in 50% MeCN-50% water (v/v) is reported. Br?nsted plots of the second-order rate constants are biphasic with an initial steep rise for weakly basic thiolate ions (beta(nuc) approximately equal to 1.0) followed by a slightly descending leg with a negative slope (beta(nuc) approximately equal to -0.2) for strongly basic thiolate ions. This indicates a change from rate-limiting leaving group departure at low pK(RSH)(a) to rate-limiting nucleophilic attachment at high pK(RSH)(a). The negative beta(nuc) values result from a combination of minimal progress of C-S bond formation at the transition state and the requirement for partial desolvation of the nucleophile before it enters the transition state. Possible factors that may affect the degree of bond formation in reactions of Fischer carbene complexes as well as reactions of other unsaturated electrophiles with thiolate ions are discussed.