One primary goat in biomimetic research is to understand mechanisms of dioxygen activation, structures of reactive intermediates, and reactivities of the intermediates involved in catalytic oxidation reactions by metalloenzymes, such as heme and nonheme iron oxygenases. In this communication, we have reported the first example of generating nonheme iron(III)-hydroperoxo and iron(IV)-oxo complexes by activating O-2 with a biologically important electron donor, an NADH analogue, and an acid. The formation of iron(ill)-hydroperoxo and iron(IV)-oxo complexes was found to depend on the supporting ligands. We have also demonstrated that high-spin nonheme iron(II) complexes with a low oxidation potential are able to bind and activate O-2 to generate, the iron-oxygen intermediates.