The neutral (mu-oxo)bis(mu-carboxylato)diiron(III) complexes [Fe2O(mu-XDK)(bpy(2)(NCS)(2)] (1), [Fe2O(mu-XDK)(bpy)(2)-((NCS)-N-15)(2)] (1a), [Fe2O(mu-XDK)(bpy)(2)(NCSe)(2)] (2), [Fe2O(mu-XDK)(bpy)(2)(N-3)(2)] (3), and [Fe2O(mu-XDK)(bpy)(2)-((NNN)-N-15)(2)] (1a), where XDK is the doubly deprotonated form of m-xylylenediamine bis(Kemp's triacid imide) and bpy is 2,2'-bipyridyl, have been synthesized, The octahedrally coordinated! iron centers are triply bridged by an oxo group and the two carboxylate groups of XDK. In the X-ray crystal structures of 1 and 2, the diiron centers have C-2 symmetry, whereas in 3 the coordinated azide ligands are also anti with respect to the Fe-O-Fe plane but in different rotational conformations with respect to the Fe-N-3 bond. The optical spectra of these highly colored compounds are characterized by broad ligand-field bands centered similar to 650 nm and a more intense combination of ligand-field and ligand-to-metal charge-transfer bands between 350 and 500 nm. Compounds 1-3 exhibit intense IR bands (similar to 2050 cm(-1)) which are characteristic of terminally bound isothiocyanate, isoselenocyanate, and azide ligands, respectively, These bands shift to lower energies upon substitution with (NCS-)-N-15 in as (Delta nu = 24 cm(-1)) and (NNN-)-N-15 in 3a (Delta nu = 8 cm(-1)), and the failure of the latter to reveal two resolved features indicates a need for caution in interpreting analogous data from isotopically substituted protein adducts. Resonance Raman experiments on 1-3 revealed bands (similar to 520 cm(-1)) attributable to the symmetric FeOFe stretching frequency. The Mossbauer spectra of 1-3 at 77 K display symmetric quadrupole doublets with Isomer shifts (similar to 0.5 mm s(-1)) and quadrupole splittings (similar to 1.7 mm s(-1)) typical of (mu-oxo)diiron(III) compounds. Crystal data: 1 . 2C(6)H(6), monoclinic, P2(1)/n, a = 19.6692(2) Angstrom, b = 14.3069(1) Angstrom, c = 22.6502(3) Angstrom, beta = 96.922(1)degrees, V = 6327.4(1) Angstrom(3), Z = 4, T = -90 degrees C; 2 . 2C(6)H(6), monoclinic, C2/c, a = 22.6834(3) Angstrom, b = 14.4315(2) Angstrom, c = 19.7013(4) Angstrom, beta = 97.837(1)degrees V = 6389.1(2) Angstrom(3), Z = 4, T = -85 degrees C; 3 . CH2Cl2 . C6H3(CH3)(3), triclinic, , a 12.1645(2) Angstrom, b = 14.9196(3) Angstrom, c = 18.0690(3) Angstrom, alpha = 79.688(1)degrees, beta = 78.611(1)degrees, gamma = 83.453(1)degrees, V = 3152.3(1) Angstrom(3), Z = 2, T = -85 degrees C.