 The Fe-only hydrogenases have a Fe2(μ-SR)2(μ-CO)(CO)2(CN)2 active site connected to a 4Fe4S cluster via a bridging thiolate.The active site of the [NiFe]-hydrogenases are described as (NC)2(OC)Fe(μ-SR)2Ni(SR)2 (where SR is cysteinyl).The “FeS-free” hydrogenases have an undetermined active site containing an Fe(CO)2 center.