Critical aspects of [NiFe]hydrogenase ligand composition.

Structural analysis of the resting state of [NiFe]hydrogenase ([NiFe]H(2)ase) shows that the active site has a characteristic bis(mu-thiolato)NiFe unit, where the Ni atom and the Fe atom are bridged by an undetermined oxygen-bearing ligand. This ligand probably derives from the aqueous solvent and is therefore most likely to be H(2)O, OH(-) or O(2-). Here, we compare the reactivities of a NiFe and a NiRu complex when bearing either acetonitrile or aqueous ligands and demonstrate the critical role of an aqueous ligand in hydrogenase and its mimics. We also make observations on the necessity of organometallic metal-carbon bonds to the supporting frameworks.