Nitric oxide (NO) is frequently used to probe the substrate–binding site of “spectroscopically silent” non-heme Fe2+ sites of metalloenzymes, such as superoxide reductase (SOR). Herein we use NO to probe the superoxide binding site of our thiolate–ligated biomimetic SOR model [FeII(SMe2N4(tren))]+ (1). Like NO–bound trans cysteinate-ligated SOR (SOR-NO), the rhombic S= 3/2 EPR signal of NO–bound cis thiolate-ligated [Fe(SMe2N4(tren)(NO)]+ (2; g = 4.44, 3.54, 1.97), isotopically sensitive νNO(ν15NO) stretching frequency (1685(1640) cm−1), and 0.05 Å decrease in Fe–S bond length are shown to be consistent with the oxidative addition of NO to Fe(II) to afford an Fe(III)–NO− {FeNO}7 species containing high–spin (S= 5/2) Fe(III) antiferromagnetically coupled to NO− (S= 1). The cis versus trans positioning of the thiolate does not appear to influence these properties. Although it has yet to be crystallographically characterized, SOR-NO is presumed to possess a bent Fe-NO similar to that of 2 (Fe–N–O= 151.7(4)°). The N–O bond is shown to be more activated in 2 relative to N– and O–ligated {FeNO}7 complexes, and this is attributed to the electron donating properties of the thiolate ligand. Hydrogen bonding to the cysteinate sulfur attenuates N–O bond activation in SOR as shown by its higher νNO frequency (1721 cm−1). In contrast, the νO–O frequency of SOR peroxo intermediate and its analogues is not affected by H-bonds to the cysteinate sulfur, or other factors influencing the Fe–SR bond strength. These only influence the νFe–O frequency. Reactions between 1 and NO2− are shown to result in the proton–dependent heterolytic cleavage of an N–O bond. The mechanism of this reaction is proposed to involve both FeII–NO2− and {FeNO}6 intermediates similar to those implicated in the mechanism of NiR–promoted NO2− reduction.