Previous studies provide evidence that [4Fe-4S] clusters serve as targets of reactive nitrogen oxide species in biology. The products of this reaction range from dinitrosyliron complexes, [Fe(NO)(2)L-2](-), to Roussin's black anion, [Fe4S3(NO)(7)](-). To date, the pathways by which these reactions occur have not been fully elucidated. In this study, we prepared the site-differentiated complexes [Fe4S4(LS3)L'](2-) (LS3 = 1,3,5-tris(4,6-dimethyl-3-mercaptophenylthio)-2,4,6-tris(p-tolylthio)benzene; L' = Cl, SEt, SPh, N3, 2-SPyr, Tp, S2CNEt2) to serve as synthetic models for biological [4Fe-4S] clusters and studied their reactivity toward NO(g) and Ph3CSNO. The products were characterized by X-ray crystallography, mass spectrometry, and IR, electron paramagnetic resonance (EPR), and H-1 NMR spectroscopy. In all cases reported here, the reactions proceed via formation of the S = 1/2 species [Fe4S4(NO)(4)](-), which ultimately converts to EPR-silent [Fe4S3(NO)(7)](-).