A unifying structural and electronic concept for Hmd and [FeFe] hydrogenase active sites.

The hydrogenases [FeFe] and Hmd feature at first sight rather different active sites. A closer inspection reveals striking similarities, which allow us to define swapped ligand spheres in such a way that the single active iron center of Hmd functions in a first-shell ligand environment resembling the reacting iron atom in [FeFe] hydrogenase and vice versa. These redesigned ligand environments can be conveniently studied with quantum chemical methods and point to general reactivity principles for iron centers with hydrogenase activity.