Decomposition of S-nitrosothiols by mercury(II) and silver salts

Rate measurements have been obtained for the reaction of a number of S-nitrosothiols (RSNO) in water with mercury(II) salts. Reaction is first order in both reactants, and the products are nitrous acid and the corresponding thiol–Hg²⁺ complex. Decomposition is ca. 10³ faster when mercury(II) nitrate, rather than mercury(II) chloride is used as the source of mercury(II). This is consistent with the fact that whereas the nitrate salt is fully ionised in water, the chloride exists almost entirely in the undissociated molecular form. There is very little variation in the rate constant with RSNO structure. Silver ion reacts similarly, although less rapidly, and the kinetic order with respect to [Ag⁺] is ca. 2. The results are discussed in terms of a mechanism involving rate-limiting attack by water, at the nitrogen atom in the mercury(II) (or silver) ion complex of the S-nitrosothiol, and are contrasted with the corresponding reactions of S-nitrosothiols with Cu⁺, which generate nitric oxide.