The heterodinuclear mixed-valence complex [FeMn(ICIMP)(OAc)₂Cl] (1) {H₂ICIMP = 2-(N-carboxylmethyl)-[N-(N-methylimidazolyl-2-methyl) aminomethyl]-[6-(N-isopropylmethyl)-[N-(N-methylimidazolyl-2-methyl)] aminomethyl-4-methylphenol], an unsymmetrical N₄O₂ donor ligand} has been synthesized and fully characterized by several spectroscopic techniques as well as by X-ray crystallography. The crystal structure of the complex reveals that both metal centers in 1 are six-coordinate with the chloride ion occupying the sixth coordination site of the Mn^II ion. The phenoxide moiety of the ICIMP ligand and both acetate ligands bridge the two metal ions of the complex. Mössbauer spectroscopy shows that the iron ion in 1 is high-spin Fe^III. Two quasi-reversible redox reactions for the complex, attributed to the Fe^IIIMn^II/Fe ^IIMn^II (at -0.67 V versus Fc/Fc⁺) and Fe ^IIIMn^II/Fe^IIIMn^III (at 0.84 V), were observed by means of cyclic voltammetry. Complex 1, with an Fe ^III-Mn^II distance of 3.58 Å, may serve as a model for the mixed-valence oxidation state of purple acid phosphatase from sweet potato. The capability of the complex to effect organophosphate hydrolysis (phosphatase activity) has been investigated at different pH levels (5.5-11) by using bis(2,4-dinitrophenyl)phosphate (BDNPP) as the substrate. Density functional theory calculations indicate that the substrate coordinates to the Mn^II ion. In the transition state, a hydroxide ion that bridges the two metal ions becomes terminally coordinated to the Fe^III ion and acts as a nucleophile, attacking the phosphorus center of BDNPP with the concomitant dissociation of the leaving group.