Spin Polarization Reveals the Coordination Geometry of the [FeFe] Hydrogenase Active Site in Its CO-Inhibited State

[Image: see text] The active site of [FeFe] hydrogenase features a binuclear iron cofactor Fe(2)ADT(CO)(3)(CN)(2), where ADT represents the bridging ligand aza-propane-dithiolate. The terminal diatomic ligands all coordinate in a basal configuration, and one CO bridges the two irons leaving an open coordination site at which the hydrogen species and the competitive inhibitor CO bind. Externally supplied CO is expected to coordinate in an apical configuration. However, an alternative configuration has been proposed in which, due to ligand rotation, the CN(–) bound to the distal Fe becomes apical. Using selective (13)C isotope labeling of the CN(–) and CO(ext) ligands in combination with pulsed (13)C electron–nuclear–nuclear triple resonance spectroscopy, spin polarization effects are revealed that, according to density functional theory calculations, are consistent with only the “unrotated” apical CO(ext) configuration.