The novel eta(2)-mu(4)-peroxo hexairon(III) complex, [Fe-6(O)(2)(O)(23)(OAC)(9)](-), was prepared as its [Fe3O(OAc)(6)-(H2O)(3)](+) salt by addition of H2O2 to basic iron acetate in the presence of NaOCH3. The compound [Fe3O(OAc)(6)-(H2O)(3)][Fe-6(O)(2)(O-2)(3)(OAc)(9)]. 8H(2)O crystallizes in the hexagonal space group, P6(3)/m with unit cell parameters of a=14.276(5), c=18.935(3) Angstrom, V=3342.0(1) Angstrom(3), and Z=2; the structure was solved and refined to a final residual of R=0.044. The anionic component of the salt contains two trinuclear {Fe3O}(7+) units arranged in a face-to-face manner. The resulting trigonal prism has eta(2)-mu(4)-peroxo ligands bridging each rectangular face and eta(2)-mu(2)-acetates bridging each edge of the rectangular and triangular faces. Two distinguishing structural features are an elongated O-O bond distance of 1.472(9) Angstrom for the bridging peroxo ligand arid significant displacemeat of this group by 1.05 Angstrom out of the plane of the four iron atoms that it links. The v(0-0) stretching mode is readily observed by Raman spectroscopy at 844 cm(-1), as in other peroxo-bridged polyiron(III) complexes, but the UV-visible spectrum lacks the distinctive charge transfer band in the 500-800 Mn region found in most other ferric peroxo species. Mossbauer isomer shift and quadrupole splitting parameters at 4.2 K are delta=0.51 and 0.53 mm/s and Delta E(q)=1.16 and 0.79 mm/s for the cation and anionic iron clusters, respectively. Asymmetric line broadening indicated the onset of slow spin relaxation at the lowest temperatures (4.2 K). Variable temperature magnetic susceptibility data measured on a solid sample of [Fe3O(OAC)(6)(H2O)(3)][Fe-6(O)(2)(O-2)(3)(OAC)(9)] revealed antiferromagnetic coupling within the {Fe3O}(7+) units (J similar to-30 to -50 cm(-1)) and weak ferromagnetic coupling (J similar to 1 cm(-1)) through the eta(2)-mu(4)-peroxo bridges, where H=-2J/S-i . S-j/.