A super-reduced diferrous [2Fe-2S] cluster.

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. 57Fe 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 ST = 0 ground state and establish a lower limit for the exchange coupling, −J ≥ 30 cm–1.