[NiFe]-hydrogenases catalyze the reversible activation of H2 using a unique NiFe(CN)2CO metal site, which is assembled by a sophisticated multiprotein machinery. The [4Fe-4S] cluster-containing HypCD complex, which possesses an ATPase activity with a hitherto unknown function, serves as the hub for the assembly of the Fe(CN)2CO subfragment. HypCD is also thought to be responsible for the subsequent transfer of the iron fragment to the apo-form of the catalytic hydrogenase subunit, but the underlying mechanism has remained unexplored. Here, we performed a thorough spectroscopic characterization of different HypCD preparations using infrared, Mössbauer, and NRVS spectroscopy, revealing molecular details of the coordination of the Fe(CN)2CO fragment. Moreover, biochemical assays in combination with spectroscopy, AlphaFold structure predictions, protein-ligand docking calculations, and crosslinking MS deciphered unexpected mechanistic aspects of the ATP requirement of HypCD, which we found to actually trigger the transfer of the Fe(CN)2CO fragment to the apo-hydrogenase.