A biomimetic [2Fe-2S] cluster has been isolated in the fully reduced diferrous form and characterized by X-ray diffraction. This completes a consistent series of synthetic analogues of protein-bound [2Fe-2S](z) redox centers (z = 2+, 1+, 0) with identical capping ligands. Fe-57 Mossbauer data of the extremely oxidation-sensitive complex compare well with those of the very few reports of all-ferrous ferredoxins and Rieske centers; they confirm the S-T = 0 ground state and establish a lower limit for the exchange coupling, -J >= 30 cm(-1).