The symmetrically ligated complexes 1, 2, and 3 with a (mu-oxo)bis(mu-acetato)diferric core can be one-electron oxidized electrochemically or chemically with aminyl radical cations [(NR3)-N-.][SbCl6] in acetonitrile yielding complexes which contain the mixed-valent {(mu-oxo)bis(mu-acetato)iron(IV)iron(III)}(3+) core: [([9]aneN(3))(2)Fe-2(III)(mu-O)(mu-CH3CO2)(2)](ClO4)(2) (1(ClO4)(2)), [(Me-3[9]aneN(3))(2)Fe-2(III)(mu-O)(mu-CH3CO2)(2)](PF6)(2) (2(PF6)(2)), and [(tpb)(2) Fe-2(III)(mu-O)(mu-CH3CO2)(2)] (3) where ([9]aneN(3)) is the neutral triamine 1,4,7-triazacy- clononane and (Me-3[9]aneN(3)) is its tris-N-methylated derivative, and (tpb)(-) is the monoanion trispyrazolylborate. The asymmetrically ligated complex [(Me-3[9]aneN(3))Fe-III(mu-O)(mu-CH3CO2)(2)Fe-III(tpb)](PF6) (4(PF6)) and its one-electron oxidized form [4(ox)](2+) have also been prepared. Finally, the known heterodinuclear species [(Me-3[9]aneN(3))Cr-III(mu-O)(mu-CH3CO2)(2)Fe([9]aneN(3))](PF6)(2) (5(PF6)(2)) can also be one-electron oxidized yielding [5(ox)](3+) containing an iron(IV) ion. The structure of 4(PF6)(.)0.5CH(3)CN(.)0.25(C2H5)(2)O has been determined by X-ray crystallography and that of [5(ox)](2+) by Fe K-edge EXAFS-spectroscopy (Fe-IV-O-oxo: 1.69(1) Angstrom; Fe-IV-O-carboxylato 1.93(3) Angstrom, Fe-IV-N 2.00(2) Angstrom) contrasting the data for 5 (Fe-III-O-oxo 1.80 Angstrom; Fe-III-O-carboxylato 2.05 Angstrom, Fe-N 2.20 Angstrom). [5(ox)](2+) has an S-t = 1/2 ground state whereas all complexes containing the mixed-valent {Fe-IV(mu-O) (mu-CH3CO2)(2)Fe-III}(3+) core have an S-t = 3/2 ground state. Mossbauer spectra of the oxidized forms of complexes clearly show the presence of low spin Fe-IV ions (isomer shift similar to0.02 mm s(-1), quadrupole splitting similar to1.4 mm s(-1) at 80 K), whereas the high spin Fe-III ion exhibits delta approximate to 0.46 mm s(-1) and DeltaE(Q) approximate to 0.5 mm s(-1). Mossbauer, EPR spectral and structural parameters have been calculated by density functional theoretical methods at the BP86 and B3LYP levels. The exchange coupling constant, J, for diiron complexes with the mixed-valent Fe-IV-Fe-III core (H = -2J (S1S2)-S-.; S-1 = 5/2; S-2 = 1) has been calculated to be -88 cm(-1) (intramolecular antiferromagnetic coupling) and for the reduced diferric form of -75 cm(-1) in reasonable agreement with experiment (J = -120 cm(-1)).