Two new types of dinucleating ligands, symmetric Me(4)-tpdp and asymmetric Me(2)-tpdp, were synthesized (Me(4)-tpdp = N,N,N’,N’-tetrakis{(6-methyl-2-pyridyl)methyl}-1,3-diaminopropan-2-olate and Me(2)-tpdp = N,N-bis-{(6-methyl-2-pyridyl)methyl}-N’,N’-bis(2-pyridylmethyl)-1,3-diaminopropan-2-olate). Their mu-alkoxo diiron(II) complexes [Fe-2(Me(4)-tpdp)(C6H5COO)(H2O)](BF4)(2) (1), [Fe-2(Me(4)-tpdp)(CF3CO2)(H2O)(2)](BF4)(2) . H2O (2), and [Fe-2(Me(2)-tpdp)(CF3CO2)](BF4)(2) . nH(2)O (3) were synthesized, and the structure of complex 1 was determined by X-ray crystallography. Complex 1 crystallizes in the monoclinic space group P2(1)/n with a = 19.257(2) Angstrom, b = 17.166(3) Angstrom, c = 12.558(2) Angstrom, beta = 96.55(1)degrees, and Z = 4. The complex has a doubly bridged structure with a mu-alkoxo of ME(4)-tpdp and mu-benzoate and contains two distinct iron centers : one five-coordinate iron center with an N3O2 donor set and one six-coordinate iron center with an N3O3 donor set with an additional coordinated water molecule. Introduction of a 6-methyl group in the bond distances. The Fe-N and Fe-O bond distances in the six-coordinate iron center are substantially longer than those in the five-coordinate iron center except for the Fe-N(tertiary amine) bonds. The Mossbauer spectrum of 1 exhibited two sets of quadrupole doublets corresponding to the two distinct iron centers. The electronic spectra of 1 revealed that dissociation of the coordinated water occurs in CH2Cl2 to form two five-coordinate iron centers, whereas in DMSO, six-coordinate iron centers are predominant due to the coordination of DMSO.Complexes 1 and 2 showed reversible oxygenation at -40 degrees C, whereas reversible oxygenation was not observed for complex 3. The thermal stability of the oxy complexes toward irreversible oxidation is highly dependent on the relative degree of steric bulkiness of the dinucleating ligands and their electron donor ability; introduction of the 6-methyl group in the pyridyl group stabilizes the oxy form toward irreversible oxidation. The resonance Raman spectrum of the oxy-1 exhibited two oxygen-isotope sensitive bands in the nu(O-O) region and two bands in the nu(Fe-O) region, The presence of the two nu(O-O) Raman bands at 918 and 891 cm(-1) is interpreted in terms of Fermi resonance, while the two nu(Fe-O) bands are assigned to the symmetric and antisymmetric combinations of Fe-O stretchings of the Fe-O-O-Fe unit. Thus the Raman spectra indicated the formation of the mu-peroxo species. The oxygen affinities (P-1/2) of the complexes were determined for the first time by spectrophotometric methods; those of 1 and 2 in CH2Cl2 at -40 degrees C are ca. 6 and ca. 41 Torr, respectively.