The reaction of [Fell(tris(2-pyridylmethyl)amine, TPA)(NCCH3)(2)](2+) with 1 equiv. peracetic acid in CH3CN at -40degreesC results in the nearly quantitative formation of a pale green intermediate with lambda(max) at 724 nm (epsilon approximate to 300 M-1-cm(-1)) formulated as [Fe-IV(O)(TPA)](2+) by a combination of spectroscopic techniques. Its electrospray mass spectrum shows a prominent feature at m/z 461, corresponding to the [Fe-IV(O)(TPA)(CIO4)](+) ion. The Mossbauer spectra recorded in zero field reveal a doublet with DeltaE(Q) = 0.92(2) mm/s and delta = 0.01(2) mm/s; analysis of spectra obtained in strong magnetic fields yields parameters characteristic of S = 1 Fe-IV==O complexes. The presence of an Fe-IV==O unit is also indicated in its Fe K-edge x-ray absorption spectrum by an intense 1-s --> 3-d transition and the requirement for an O/N scatterer at 1.67 Angstrom to fit the extended x-ray absorption fine structure region. The [Fe-IV(O)(TPA)](2+) intermediate is stable at -40degreesC for several days but decays quantitatively on warming to [Fe-2(mu-O)(mu-OAc)(TPA)(2)](3+). Addition of thioanisole or cycloodene at -40degreesC results in the formation of thioanisole oxide (100% yield) or cycloodene oxide (30% yield), respectively; thus [Fe-IV(O)(TPA)](2+) is an effective oxygen-atom transfer agent. It is proposed that the Fe-IV=O species derives from O-O bond heterolysis of an unobserved Fe-II(TPA)-acyl peroxide complex. The characterization of [Fe-IV(O)(TPA)](2+) as having a reactive terminal Fe-IV==O unit in a nonheme ligand environment lends credence to the proposed participation of analogous species in the oxygen activation mechanisms of many mononuclear nonheme iron enzymes.