Thermodynamic Hydricity of [FeFe]-Hydrogenases

After remaining elusive for many years, terminal hydride states have now been identified in several native and mutant [FeFe]-hydrogenases. In this Perspective, hydride states of [FeFe]-hydrogenases are considered on the basis of hydricity, a thermodynamic parameter that has been instrumental for the rational design of synthetic molecular catalysts. Importantly, it is shown how the hydricity of [FeFe]-hydrogenases can inspire future research efforts in both the study of hydrogenases and the design of molecular catalysts. By using hydricity, quantitative linear free energy relationships can be developed to relate the driving force and rate of different [FeFe]-hydrogenase variants. Further development of these correlations across a wide range of hydrogenase variants can potentially lead to new insights into the structure-activity relationships of [FeFe]-hydrogenases. The hydricity of [FeFe]-hydrogenase is also compared to select transition metal hydride complexes, which emphasizes the strong electronic communication between the diiron active site and the extended protein scaffold of the enzyme. A mechanism is proposed for how key hydrogen bonding interactions might affect the hydricity of [FeFe]-hydrogenases, providing a basis for the emulation of these structural features in synthetic molecular complexes.