Hydrogenase enzymes catalyze the rapid and reversible interconversion of H₂ with protons and electrons. The active site of the [FeFe] hydrogenase is the H cluster, which consists of a [4Fe–4S]_H subcluster linked to an organometallic [2Fe]_H subcluster. Understanding the biosynthesis and catalytic mechanism of this structurally unusual active site will aid in the development of synthetic and biological hydrogenase catalysts for applications in solar fuel generation. The [2Fe]_H subcluster is synthesized and inserted by three maturase enzymesHydE, HydF, and HydGin a complex process that involves inorganic, organometallic, and organic radical chemistry. HydG is a member of the radical S-adenosyl-l-methionine (SAM) family of enzymes and is thought to play a prominent role in [2Fe]_H subcluster biosynthesis by converting inorganic Fe²⁺, l-cysteine (Cys), and l-tyrosine (Tyr) into an organometallic [(Cys)Fe(CO)₂(CN)]⁻ intermediate that is eventually incorporated into the [2Fe]_H subcluster. In this Forum Article, the mechanism of [2Fe]_H subcluster biosynthesis is discussed with a focus on how this key [(Cys)Fe(CO)₂(CN)]⁻ species is formed. Particular attention is given to the initial metallocluster composition of HydG, the modes of substrate binding (Fe²⁺, Cys, Tyr, and SAM), the mechanism of SAM-mediated Tyr cleavage to CO and CN^–, and the identification of the final organometallic products of the reaction.