The synthesis and characterization of novel tetranuclear and octanuclear iron(III) complexes with structures based on a nearly square arrangement of four iron ions are reported. Reaction of ferric nitrate, Sodium acetate, and the unsymmetrical binucleating ligand HBMDP, where HBMDP is N,N,N'-tris((N-methyl)-2-benzimidazolylmethy)-N'-methyl-1,3-diamino-2-propanol, in acetone/water yields the tetranuclear iron complex [Fe-4(mu-O)(2)(mu-BMDP)(2)(mu-OAc)(2)](4+), which exhibits coordination number asymmetry. The structure of [Fe-4(mu-O)(2)(mu-BMDP)(2)(mu-OAc)(2)](NO3)(3)(OH). 12H(2)O has been determined by single-crystal X-ray diffraction. Each (mu-BMDP) ligand spans two iron(III) ions and causes these ions to become structurally distinct. Within this binuclear unit one iron atom is five-coordinate with distorted square pyramidal geometry and an N2O3 donor set, while the other iron is six-coordinate with distorted octahedral geometry and an N3O3 donor set. Two of these binuclear units are Linked through a pair of oxo and acetato bridges to form the centrosymmetric tetranuclear complex. The coordinatively nonequivalent iron atoms exhibit distinct Mossbauer spectroscopic parameters and produce a pair of doublets at 80 K. The iron(III) centers are coupled antiferromagnetically with a room-temperature moment of 1.9 mu(B) per iron with J = -103.3 cm(-1), zJ' = -105.9 cm(-1). The properties of the unsymmetric cation [Fe-4(mu-O)(2)(mu-BMDP)(2)(mu-OAc)(2)](4+) are similar to those observed for binuclear iron proteins with comparable coordinative inequivalence. Efforts to increase the solubility of [Fe-4(mu-O)(2)(mu-BMDP)(2)(mu-OAc)(2)](4+) by metathesis with sodium tetrafluoroborate resulted in the isolation of crystals of a new octanuclear iron species, [Fe-8(mu-O)(4)(mu-BMDP)(4)(OH)(4)(mu-OAc)(4)](BF4)(3)(OH). 2CH(3)CN . 8H(2)O (2), which has also been characterized by single-crystal X-ray diffraction. The asymmetric unit consists of an Fe-2(mu-O)(mu-BMDP)(mu-OAc) (OH) group which is externally bridged via the oxo ions to form a molecular square with four of the eight iron ions at the corners. Both iron sites are six-coordinate with distorted octahedral geometry. One has an N2O4 donor set; the other has an N3O3 donor set.