In contrast to the four-coordinate tetrahedral {Fe(NO)(2)}(9) DNICs with an EPR g value of 2.03, the newly synthesized nonclassical six-coordinate {Fe(NO)(2)}(9) DNIC [(1-Melm)(2)(eta(2)-ONO)Fe(NO)(2)] (1-Melm = 1-methytimidazole) (1) displays an EPR signal g = 2.013. The tempera tu re-dependent reversible transformation occurs between the six-coordinate chelating nitrito {Fe(NO)(2)}(9) DNIC 1 and the four-coordinate monodentate nitrito {Fe(NO)(2)}(9) DNIC [(1-Melm)(ONO)Fe(NO)(2)] (2-Melm). The chelating nitrito of DNIC 1, triggered by PPh3, undergoes O-atom transfer to yield O=PPh3, accompanied by reductive elimination of NO and the generation of {Fe(NO)(2)}(10) DNIC [(1-Melm)(PPh3)Fe(NO)(2)] (3), in contrast to the inertness of the nitrite-containing {Fe(NO)(2)}(9) DNIC [(Hlm)(ONO)Fe(NO)(2)] (Hlm = imidazole) (2-Hlm) toward PPh3. The findings, EPR signals of g = 2.013 for complex 1 and g = 2.03 for complexes 2-Melm/2-Hlm, imply that characterization of DNICs may be possible via their distinctive EPR signal g = 2.03 for the tetrahedral DNICs and EPR signal g = 2.01 for the six-coordinate DNICs. This study also implicates that the six-coordinate nitrite-containing {Fe(NO)(2)}(9) DNICs may act as a transient intermediate (or an active center) to trigger the transformation of nitrite into nitric oxide