Nitrosylation of high-spin [Fe(kappa(2)-O2NO)(4)](2-) (1) yields {Fe(NO)}(7) mononitrosyl iron complex (MNIC) [(kappa(2)-O2NO)(kappa(1)-ONO2)(3)Fe(NO)](2-) (2) displaying an S = 3/2 axial electron paramagnetic resonance (EPR) spectrum (g(perpendicular to) = 3.988 and g(parallel to) = 2.000). The thermally unstable nitrate-containing {Fe(NO)(2)}(9) dinitrosyl iron complex (DNIC) [(kappa(1)-ONO2)(2)Fe(NO)(2)](-) (3) was exclusively obtained from reaction of HNO3 and [(OAc)(2)Fe(NO)(2)](-) and was characterized by IR, UV-vis, EPR, super-conducting quantum interference device (SQUID), X-ray absorption spectroscopy (XAS), and single-crystal X-ray diffraction (XRD). In contrast to {Fe(NO)(2)}(9) DNIC [(ONO)(2)Fe(NO)(2)](-) constructed by two monodentate O-bound nitrito ligands, the weak interaction between Fe(1) and the distal oxygens O(5)/O(7) of nitrato-coordinated ligands (Fe(1)center dot center dot center dot O(5) and Fe(1)center dot center dot center dot O(7) distances of 2.582(2) and 2.583(2) angstrom, respectively) may play important roles in stabilizing DNIC 3. Transformation of nitrate-containing DNIC 3 into N-bound nitro {Fe(NO)}(6) [(NO)(kappa 1-NO2)Fe(S2CNEt2)(2)] (7) triggered by bis(diethylthiocarbamoyl) disulfide ((S2CNEt2)(2)) implicates that nitrate-to-nitrite conversion may occur via the intramolecular association of the coordinated nitrate and the adjacent polarized NO-coordinate ligand (nitrosonium) of the proposed {Fe(NO)(2)}(7) intermediate [(NO)(2)(kappa(1)-ONO2)Fe(S2CNEt2)(2)] (A) yielding {Fe(NO)}(7) [(NO)Fe(S2CNEt2)(2)] (6) along with the release of N2O4 (center dot NO2) and the subsequent binding of center dot NO2 to complex 6. The N-bound nitro {Fe(NO)}(6) complex 7 undergoes Me2S-promoted O-atom transfer facilitated by imidazole to give {Fe(NO)}(7) complex 6 accompanied by release of nitric oxide. This result demonstrates that nitrate-containing DNIC 3 acts as an active center to modulate nitrate-to-nitrite-to-nitric oxide conversion.