Thionitrites

The red [SSNO] anion (37) is produced by the reaction of an ionic nitrite with elemental sulfur or a polysulfide in acetone, DMF or DMSO.115 The [SSNO] anion has a planar cis structure with a short S-S distance (1.99 A), and S-N and N-O distances of 1.67 and 1.22 A, respectively. The treatment of [SSNO]- with triphenylphosphine produces the thionitrite anion [SNO]- (38) with a bond angle of ca. 120° at nitrogen and S-N and N-O bond lengths of 1.69 and 1.21 A, respectively. 

Compounds of this structure were first described many years ago by a Scottish chemist named MacBeth. He noted that addition of nitrite to an acid solution of a thiol produced a transient red color, which he credited to formation of an S-nitrosothiol or thionitrite. The chemical properties of these compounds have been described elsewhere in this volume by Mutus (Chapter 4). There are also reviews by Williams [11] and by Wang et al. [12]. All known compounds of this class decompose in what appears to be a spontaneous manner as follows ... 

Chemical/Physical. In the presence of nitric oxide, ethyl mercaptan reacted with OH radicals forming ethyl thionitrite. The rate constant for this reaction is 2.7 x 10 " at 20 °C (MacLeod et al, 1984). 

Methyl thionitrite, see Methyl mercaptan Methylthiopropene, see Aldicarb A-Methyl-N-(3- (trifluoromethy 1) phenyl)urea, see Fluometuron... 

In scrupulously anaerobic basic solutions, NO can combine directly with thiolates to form thionitrites (S-nitrosothiols) [25], which can decay by several modes to yield NO. 

Thiolates reduce nitroprusside stoichiometrically to yield R2S2 and [Fe(CN)s(NO)] , which can then convert to [Fe(CN)4(NO)] with subsequent release of NO. Reduction may take place through thionitrito adduct formation (see below) followed by generation of the redox products and eventually [Fe(CN)e] or [Fe(CN)5L]" or release of the thionitrite ligand with subsequent decay to the same products [7, 54]. If oxygen is present, nitroprusside is regenerated and can catalyze the air oxidation of thiols, such as cysteine to cystine [91]. 

Sulfurdiimines RN=S=NR and thionitrites RN=S=0 (R = alkyl, aryl) have been relatively little investigated as ligands in metal complexes. This lack of attention has been little deserved as both types of ligands have interesting ambident coordination properties, since they may bind via N, S, O, the 7t-N=S or jr-S=0 bond for example. They may be compared with allenes and naturally also with the isostructural and the isoelectronic sulfur dioxide 0=S—O.13 We will first discuss some relevant data concerning RNSNR and RNSO and subsequently the properties of the metal complexes. 

Coupling of thiols. Reaction of thiols with Fe(NO,)i supported on Clayfen results in coupling to disulfides via a thionitrite, RSNO. Yields are —40-95%. 

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