The reaction of [Fe(II)(tris(2-pyridylmethyl)amine, TPA)(NCCH(3))(2)](2+) with 1 equiv. peracetic acid in CH(3)CN at -40 degrees C results in the nearly quantitative formation of a pale green intermediate with lambda(max) at 724 nm ( epsilon approximately 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 mz 461, corresponding to the [Fe(IV)(O)(TPA)(ClO(4))](+) ion. The Mössbauer spectra recorded in zero field reveal a doublet with DeltaE(Q) = 0.92(2) mms and delta = 0.01(2) mms; 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 ON scatterer at 1.67 A to fit the extended x-ray absorption fine structure region. The [Fe(IV)(O)(TPA)](2+) intermediate is stable at -40 degrees C for several days but decays quantitatively on warming to [Fe(2)(mu-O)(mu-OAc)(TPA)(2)](3+). Addition of thioanisole or cyclooctene at -40 degrees C results in the formation of thioanisole oxide (100% yield) or cyclooctene 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 OO 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.