Three maturase enzymes-HydE, HydF, and HydG-synthesize and insert the organometallic component of the [FeFe]-hydrogenase active site (the H-cluster). HydG generates the first organometallic intermediates in this process, ultimately producing an [Fe(CO)(2)(CN)] complex. A limitation in understanding the mechanism by which this complex forms has been uncertainty regarding the precise metallocluster composition of HydG that comprises active enzyme. We herein show that the HydG auxiliary cluster must bind both L-cysteine and a dangler Fe in order to generate the [Fe(CO)(2)(CN)] product. These findings support a mechanistic framework in which a [(Cys)Fe(CO)(2)(CN)](-) species is a key intermediate in H-cluster maturation.