A reversible redox reaction ({Fe(NO)(2)}(9) DNIC [(NO)(2)Fe(N(Mes)(TMS))(2)](-) (4) reversible arrow oxidized-form DNIC [(NO)(2)Fe(N(Mes)(TMS))(2)] (5) (Mes = mesityl, TMS = trimethylsilane)), characterized by IR, UV-vis, H-1/N-15 NMR, SQUID, XAS, single-crystal X-ray structure, and DFT calculation, was demonstrated. The electronic structure of the oxidized-form DNIC 5 (S-total = 0) may be best described as the delocalized aminyl radical [(N(Mes)(TMS))(2)](2)(-center dot) stabilized by the electron-deficient {Fe-III(NO-)(2)}(9) motif, that is, substantial spin is delocalized onto the [(N(Mes)(TMS))(2)](2)(-center dot) such that the highly covalent dinitrosyl iron core (DNIC) is preserved. In addition to IR, EPR (g approximate to 2.03 for {Fe(NO)(2)}(9)), single-crystal X-ray structure (Fe-N(O) and N-O bond distances), and Fe K-edge pre-edge energy (7113.1-7113.3 eV for {Fe(NO)(2)}(10) vs 7113.4-7113.9 eV for {Fe(NO)(2)}(9)), the N-15 NMR spectrum of [Fe((NO)-N-15)(2)] was also explored to serve as an efficient tool to characterize and discriminate {Fe(NO)(2)}(9) (delta 23.1-76.1 ppm) and {Fe(NO)(2)}(10) (delta -7.8-25.0 ppm) DNICs. To the best of our knowledge, DNIC 5 is the first structurally characterized tetrahedral DNIC formulated as covalent-delocalized [{Fe-III(NO-)(2)}(9)-[N(Mes)(TMS)](2)(-center dot)]. This result may explain why all tetrahedral DNICs containing monodentate-coordinate ligands isolated and characterized nowadays are confined in the {Fe(NO)(2)}(9) and {Fe(NO)(2)}(10) DNICs in chemistry and biology.