The heterodinuclear complexes [(FeMnII)-Mn-III(L-Bn)(mu-OAc)(2)](ClO4)(2) (1) and [(FeMnII)-Mn-III(L-Bn)(mu-OAc)(2)](ClO4) (2) with the unsymmetrical dinucleating ligand HL-Bn {[2-bis[(2-pyridylmethyl)aminomethyl]]-6-[benzyl-2-(pyridylmethyl)-aminomethyl]-4-methylphenol} were synthesized and characterized as biologically relevant models of the new Fe/Mn class of nonheme enzymes. Crystallographic studies have been completed on compound 1 and reveal an (FeMnII)-Mn-III mu-phenoxobis(mu-carboxylato) core. A single location of the Fe-III ion in 1 and of the Fe-II ion in 2 was demonstrated by Mossbauer and H-1 NMR spectroscopies, respectively. An investigation of the temperature dependence of the magnetic susceptibility of I revealed a moderate antiferromagnetic interaction (J = 20 cm(-1)) between the high-spin Fe-III and Mn-II ions in 1, which was confirmed by Mossbauer and electron paramagnetic resonance (EPR) studies. The electrochemical properties of complex I are described. A quasireversible electron transfer at -40 mV versus Ag/AgCl corresponding to the (FeMnII)-Mn-III/(FeMnII)-Mn-II couple appears in the cyclic voltammogram. Thorough investigations of the Mossbauer and EPR signatures of complex 2 were performed. The analysis allowed evidencing of a weak antiferromagnetic interaction (J. = 5.72 cm(-1)) within the (FeMnII)-Mn-II pair consistent with that deduced from magnetic susceptibility measurements (J = 6.8 cm(-1)). Owing to the similar value of the Fe-II zero-field splitting (D-Fe = 3.55 cm(-1)), the usual treatment within the strong exchange limit was precluded and a full analysis of the electronic structure of the ground state of complex 2 was developed. This situation is reminiscent of that found in many diiron and iron-manganese enzyme active sites.