Metrics & MoreArticle Recommendations * sı Supporting Information ABSTRACT: [FeFe]-hydrogenases employ a catalytic H-cluster, consisting of a [4Fe-4S] H cluster linked to a [2Fe] H subcluster with CO, CN − ligands, and an azadithiolate bridge, which mediates the rapid redox interconversion of H + and H 2 . In the biosynthesis of this H-cluster active site, the radical S-adenosyl-L-methionine (radical SAM, RS) enzyme HydG plays the crucial role of generating an organometallic [Fe(II)(CN)(CO) 2 (cysteinate)] −product that is en route to forming the H-cluster. Here, we report direct observation of this diamagnetic organometallic Fe(II) complex through Mossbauer spectroscopy, revealing an isomer shift of δ = 0.10 mm s −1 and quadrupole splitting of ΔE Q = 0.66 mm s −1 . These Mossbauer values are a change from the starting values of δ = 1.15 mm s −1 and ΔE Q = 3.23 mm s −1 for the ferrous "dangler" Fe in HydG. These values of the observed product complex B are in good agreement with Mossbauer parameters for the low-spin Fe 2+ ions in synthetic analogues, such as 57 Fe Syn-B, which we report here. These results highlight the essential role that HydG plays in converting a resting-state high-spin Fe(II) to a low-spin organometallic Fe(II) product that can be transferred to the downstream maturase enzymes.