New asymmetrical ligands (H2L) have been synthesized to provide both a bridging and a terminal phenolate to a pair of iron ions in order to mimic the binding of a single terminal tyrosinate at the diiron center of the purple acid phosphatases. H(2)L1 is 2-[(bis(2-pyridylmethyl)amino)methyl]-6-[((2-pyridylmethyl)(2-phenol)amino of methyl]-4-methylphenol and H2L'1 and H(2)L2 are obtained by replacing the a-phenol group by the 5-nitro-2-phenol and the 6-methyl-2-phenol residues, respectively, A series of mixed valence diiron complexes [(FeFeL)-Fe-II-L-III(X)(2)](Y) have been obtained where (X)(2) is the dianion of m-phenylenedipropionate or (H2PO4)(2) and Y = BPh4 or PF6 (L = L1, Ba (X)(2) mpdp, Y = BPh4, db: (X)(2) = (OAC)(2), Y = BPh4, Ic: (X)(2) = (OBZ)(2), Y = BPh4, Id: (X)(2) = (H2PO4)(2), Y = PF6; L = L'1: 1'a (X)(2) = mpdp, Y = BPh4; L = L2: 2c: (X)(2) = (OBz)(2), Y = BPh4, 2d: (X)(2) = (H2PO4)(2), Y = PF6. Diferric complexes have been obtained also either by direct synthesis or by iodine oxidation of the mixed valence precursor (L = L1, 3a (X)(2) = mpdp, Y = BPh4, 3d: (X)(2) = (H2PO4)(2), Y = PF6; L = L2, 4d: (X)(2) = (H2PO4)(2). Y PF6. Complex la [(FeFeL)-Fe-II-L-III(mpdp)](BPh4) has been characterized by X-ray diffraction techniques, la crystallizes in the monoclinic space group P21/a with the following unit cell parameters: a 22.038 (9) Angstrom, b = 16.195 (8) Angstrom, c = 16.536 (7) Angstrom, beta = 97.26 (1)degrees, Z = 4, The significant differences in the Fe-O bond lengths indicate that the metal centers are ordered. The complexes have been studied by electronic spectral, resonance Raman, magnetic susceptibility, Mossbauer, NMR, and electrochemical techniques, Mossbauer and NMR spectroscopies concur to probe that the valences of the mixed valence compounds are trapped in solution as well as in the solid state at room temperature. The electronic spectrum of the mixed-valence compounds are dominated by a charge transfer transition in the 400-600 nm domain which moves to the 550-660 nm range upon oxidation to the diferric state. In addition they exhibit a weak and broad intervalence transition close to 1100 nm. Electrochemical studies show that the systems exist in the three redox states Fe-II/Fe-II/(FeFeIII)-Fe-II/(FeFeIII)-Fe-III. Moreover they show that the introduction of the terminal phenol group results in a thermodynamic destabilization of the diferrous state higher than the stabilization of the diferric state. An expanded stability domain of the mixed valence state is therefore observed which is probably due mostly to the asymmetry of the compounds, in addition a chemical destabilization of the reduced state of la, Ic, and 1'a is observed, Comparison of the carboxylate and phosphate derivatives leads to attribute it to the partial dissociation in solution of the carboxylate oxygen trans to the phenolate. The latter feature bears an intrinsic resemblance with the dissociation of iron which is observed when purple acid phosphatases are reduced by dithionite. These studies clearly show the importance of tyrosine binding on the redox properties of the PAP enzymes. This questions the relationship of such a redox specificity with a hydrolytic function and raises the possibility that the latter may be redox regulated or that another (redox based) function is actually involved, possibly in line with the ability of the enzymes to react with peroxides.