The FeMo cofactor is a cluster with composition FeMoSC. Fe is the chemical symbol for the element iron (ferrum), and Mo is the symbol for molybdenum. This large cluster can be viewed as two subunits composed of one FeS (iron(III) sulfide) cluster and one MoFeS cluster. The two clusters are linked by three sulfide ligands and a bridging carbon atom. The unique iron (Fe) is anchored to the protein by a cysteine. It is also bound to three sulfides, resulting in tetrahedral molecular geometry. The additional six Fe centers in the cluster are each bonded to three sulfides. These six internal Fe centers define a trigonal prismatic arrangement around a central carbide center. The molybdenum is attached to three sulfides and is anchored to the protein by the imidazole group of a histidine residue. Also bound to Mo is a bidentate homocitrate cofactor, leading to octahedral geometry. Crystallographic analysis of the MoFe protein initially revealed the geometry and chemical composition of FeMoco, later confirmed by extended X-ray absorption fine-structure (EXAFS) studies. The Fe-S, Fe-Fe and Fe-Mo distances were determined to be 2.32, 2.64, and 2.73 Å respectively.