Mononuclear iron(III) species with end-on and side-on peroxide have been proposed or identified in the catalytic cycles of the antitumor drug bleomycin and a variety of enzymes, such as cytochrome P450 and Rieske dioxygenases, Only recently have biomimetic analogues of such reactive species been, generated and characterized at low temperatures. We report the synthesis and characterization of a series of iron(II) complexes with pentadentate N5 ligands that react with H2O2 to generate transient low-spin Fe-III-OOH intermediates. These intermediates have low-spin iron(III) centers exhibiting hydroperoxo-to-iron(III) charge-transfer bands in the 500-600-nm region. Their resonance Raman frequencies, nu(O-O), near 800 cm(-1) are significantly lower than those observed for high-spin counterparts. The hydroperoxo-to-iron(III) charge-transfer transition blue-shifts and the nu(O-O) of the Fe-OOH unit decreases as the N5 ligand becomes more electron donating. Thus, increasing electron density at the low-spin Fe(III) center weakens the O-O bond, in accord with conclusions drawn from published DFT calculations, The parent [(N4Py)Fe-III(eta(1)-OOH)](2+) (1a) ion in this series (N4Py = NN-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) can be converted to its conjugate base, which is demonstrated to be a high-spin iron(III) complex with a side-on peroxo ligand, [(N4Py)Fe-III(eta(2)-O-2)](+) (1b). A detailed analysis of 1a and 1b by EPR and Mossbauer spectroscopy provides insights into their electronic properties. The orientation of the observed Fe-57 A-tensor of 1 a can be explained with the frequently employed Griffith model provided the rhombic component of the ligand field, determined by the disposition of the hydroperoxo ligand, is 45degrees rotated relative to the octahedral field. EXAFS studies of la and 1b reveal the first metrical details of the iron-peroxo units in this family Of complexes: [(N4Py)Fe-III(eta(1)-OOH)](2+) has an Fe-O bond of 1.76 Angstrom, while [(N4Py)Fe-III(eta(2)-O-2)]- has two Fe-O bonds of 1.93 Angstrom, values which are in very good agreement with results obtained from DFT calculations.