[FeFe]-Hydrogenases contain a H-2-converting cofactor (H-cluster) in which a canonical [4Fe-4S] cluster is linked to a unique diiron site with three carbon monoxide (CO) and two cyanide (CN-) ligands (e.g., in the oxidized state, Hox). There has been much debate whether reduction and hydrogen binding may result in alternative rotamer structures of the diiron site in a single (Hred) or double (Hsred) reduced H-cluster species. We employed infrared spectro-electrochemistry and site-selective isotope editing to monitor the CO/CN- stretching vibrations in [FeFe]-hydrogenase HYDA1 from Chlamydomonas reinhardtii. Density functional theory calculations yielded vibrational modes of the diatomic ligands for conceivable H-cluster structures. Correlation analysis of experimental and computational IF. spectra has facilitated an assignment of Hred and Hsred to structures with a bridging hydride at the diiron site. Pronounced ligand rotation during mu H binding seems to exclude Hred and Hsred as catalytic intermediates. Only states with a conservative H-cluster geometry featuring a mu CO ligand are likely involved in rapid H-2 turnover.