σ-Bonded organotransition-metal ions. Part III. Displacement of chromium by mercury: five-co-ordinate chromium(III) as a leaving group

Abstract

The kinetics of the reaction of mercuric chloride with six penta-aquopyridiomethylchromium(III) ions in aqueous solution have been investigated. The changes in rate in the presence of added chloride ion are ascribed to reaction through the ions HgCl₃^– and HgCl₄^2– for which rate coefficients have been calculated. The products of these reactions are the corresponding pyridiomethylmercuric chloride ions and the hexa-aquochromium(III) ion together with chloropenta-aquochromium(III) ion at high chloride-ion concentrations. The formation of these chromic ions can be accommodated on the basis of the capture of a five-co-ordinate chromium(III) ion by chloride ion or solvent water. This, together with the kinetic form of the reactions and the order of reactivity of the pyridiomethylchromium(III) ions (4- > 3- > 2-), shows that the reactions involve the direct (S_E2) displacement of chromium(III) by mercury(II).

Similar reactions have been studied with mercuric bromide, which, in the presence of added bromide ion, also reacts through the ions HgBr₃^– and HgBr₄^2–, and with mercuric nitrate and acetate.