 The [2Fe] sub-site is coordinated by several CO and CN ligands and an azadithiolate bridge that allows for proton shuttling.The oxidized state of the sub-site, abbreviated as Hox, includes a paramagnetic, mixed-valence [Fe(I)Fe(II)] moiety and a diamagnetic, oxidized [4Fe4S] 2+ cluster.The single electron reduction of Hox yields two protomers that differ in the localization of the added electron and proton.