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Reactions of the [Fe(CN) 5 NO] 2À complex with biologically relevant thiols (H n RS ¼ cysteine, N-acetylcysteine, ethyl cysteinate and glutathione) are initiated by the nucleophilic attack of a thiolate (RS nÀ ) on the N atom of the NO + ligand in the complex to form [Fe(CN) 5 N(O)SR] (n+2)À . The N-S bond in the latter complex is, however, weak and can undergo both heterolytic and homolytic splitting. The former process makes the synthesis reaction reversible, whereas the latter is responsible for the spontaneous redox decomposition:The rate of the monomolecular reaction is controlled by an inductive effect in the thiol with an additional stabilisation coming from formation of a six-membered ring in the case of the N-acylated compounds. In the presence of thiolate excess, the RS (nÀ1)À radicals are transformed into the more stable RSSR (2nÀ1)À radicals, which are scavenged by both [Fe(CN) 5 N(O)SR] (n+2)À and [Fe(CN) 5 NO] 2À . The former reaction initiates, whereas the latter terminates, chain reactions of the catalysed redox decomposition. The catalytic decomposition (in the thiol excess) is much faster than the spontaneous decay (in the nitroprusside excess) but leads to the same final products. The Fe(I) reduction product is identified by UV/Vis, IR, electrochemical and EPR methods. The effect of molecular oxygen is investigated and explained. The mechanism is interpreted in terms of intermediate [Fe(CN) 5 N(O)(SR) 2 ] (2n+2)À complex formation via nucleophilic attack and its decay mainly via homolytic splitting of the N-S bond. To verify the mechanism, a simple reaction model is constructed, based on the assumption that the RSNO (nÀ1)À ligands are mostly responsible for the [Fe(CN) 5 N(O)(SR)] (n+2)À reactivity and their electronic properties are discussed within the DFT framework.

The article provides a comprehensive view of S-nitrosothiols, chemical behaviour, the pathways leading to their synthesis, their spectral properties, analytical methods of detection and determination, chemical and photochemical reactivity, kinetic aspects and suggested mechanisms. The structure parameters of S-nitrosothiols and the parent thiols are analysed with respect to their effect on the strengthening or weakening the S–NO bond, and in consequence on the S-nitrosothiol stability. This depends also on the ease of S–S bond formation in the product disulphide. These structural features seem to be crucial both to spontaneous as well as to Cu-catalysed decomposition. Principal emphasis is given here to the S-nitrosothiols’ ability to act as ligands and to the effect of coordination on the ligand properties. The chemical and photochemical behaviours of the complexes are described in more detail and their roles in chemical and biochemical systems are discussed. The aim of the article is to demonstrate that the contribution of S-nitrosothiols to chemical and biochemical processes is more diverse than supposed hitherto. Nevertheless, their role is predictable and, based on the correlation between structure and reactivity, many important mechanisms of biochemical processes can be interpreted and various applications designed.

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