Three polyiron complexes of general formula [Fe(16)MO(10)(OH)(10)(O(2)CPh)(20)], where M = Mn (1), Fe (2), or Co (3); have been synthesized from the triiron precursors [Fe3O(O(2)CPh)(6)(H2O)2(MeCN)] and [Fe3O(O(2)CPh)(6)(Me(2)O)(3)](O(2)CPh). The heptadecanuclear compounds crystallize from acetonitrile/water solutions in low to modest yields, the major product being the known undecanuclear cluster [Fe11O6(OH)(6)(O(2)CPh)(15)]. Compounds 1-3 have been characterized by single-crystal X-ray structure determinations. In each case, the heteroatom M is located on a center of symmetry within the compact Fe(16)MO(10)(OH)(10) cluster, which is; in turn, encapsulated by the 20 coordinated benzoate anions. The polymetallic clusters in the heptadecanuclear complexes can be formally constructed by fusion of two fragments generated from the {Fe11O6(OH)(6)} core of the undecanuclear molecule. All the metal ions have distorted octahedral geometry. Low-temperature Mossbauer spectroscopic and magnetic studies revealed properties normally associated with solid-state infinite arrays so that these complexes may be described as straddling the molecular/solid-state boundary. Specifically, at low temperature, both 1 and 2 exhibit Mossbauer spectra in zero and externally applied fields consistent with the onset of cooperative magnetic interactions arising from short-range ordering within the multinuclear spin structure. In addition, there is a rapid decrease in magnetic moment at low temperature, indicating that the magnetic interactions are antiferromagnetic in nature. Similar properties have been reported for polynuclear Fe(III)-apoferritin complexes formed at the initial stage of iron nucleation on the surface of the ferritin cavity.