The simplest polymetallic system, the [FeS] cluster, is constituted by two iron ions bridged by two sulfide ions and coordinated by four cysteinyl ligands (in FeS ferredoxins) or by two cysteines and two histidines (in Rieske proteins). The oxidized proteins contain two Fe ions, whereas the reduced proteins contain one Fe and one Fe ion.  These species exist in two oxidation states, (Fe) and FeFe. CDGSH iron sulfur domain is also associated with 2Fe-2S clusters.
The Rieske proteins contain Fe–S clusters that coordinate as a 2Fe–2S structure and can be found in the membrane bound cytochrome bc1 complex III in the mitochondria of eukaryotes and bacteria. They are also a part of the proteins of the chloroplast such as the cytochrome bf complex in photosynthetic organisms. These photosynthetic organisms include plants, green algae, and cyanobacteria, the bacterial precursor to chloroplasts. Both are part of the electron transport chain of their respective organisms which is a crucial step in the energy harvesting for many organisms.