[FeFe]-hydrogenases catalyze H₂ evolution at extraordinary rates at low overpotential using base metal (not platinum). In the field of renewable energy, the biosynthesis of these enzymes’ active sites, the H-cluster, is of intense interest. Among its many cofactors, the azadithiolate is unique, and its biosynthesis remains enigmatic. Three Fe–S proteins, HydE, HydF, and HydG, are essential for the H-cluster bioassembly. This paper describes an in vitro assembly approach where a synthetic [Fe(cysteine)(CO)₂(CN)] complex, “syn-B,” allows HydG- free biosynthesis of the active enzyme.