The heterodinuclear complexes [Fe(III)Mn(II)(L-Bn)(μ-OAc)(2)](ClO(4))(2) (1) and [Fe(II)Mn(II)(L-Bn)(μ-OAc)(2)](ClO(4)) (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 Fe(III)Mn(II)μ-phenoxobis(μ-carboxylato) core. A single location of the Fe(III) ion in 1 and of the Fe(II) ion in 2 was demonstrated by Mössbauer and (1)H NMR spectroscopies, respectively. An investigation of the temperature dependence of the magnetic susceptibility of 1 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 Mössbauer and electron paramagnetic resonance (EPR) studies. The electrochemical properties of complex 1 are described. A quasireversible electron transfer at -40 mV versus Ag/AgCl corresponding to the Fe(III)Mn(II)/Fe(II)Mn(II) couple appears in the cyclic voltammogram. Thorough investigations of the Mössbauer and EPR signatures of complex 2 were performed. The analysis allowed evidencing of a weak antiferromagnetic interaction (J = 5.72 cm(-1)) within the Fe(II)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.