Emerging Implications of the Concept of Hydricity in Energy-Relevant Catalytic Processes.

The "hydricity" of a species refers to its hydride-donor ability. Similar to how the pKa is useful for determining the extent of dissociation of an acid, the hydricity plays a vital role in understanding hydride-transfer reactions. A large number of transition-metal-catalyzed processes involve the hydride-transfer reaction as a key step. Among these, two key reactions-proton reduction to evolve H2 and hydride transfer to CO2 to generate formate/formic acid-represent a promising solution to build a sustainable and fossil-fuel-free energy economy. Therefore, it is imperative to develop an in-depth relationship between the hydricity of transition-metal hydrides and its influencing factors, so that efficient and suitable hydride-transfer catalysts can be designed. Moreover, such profound knowledge can also help in improving existing catalysts, in terms of their efficiency and working mechanism. With this broad aim in mind, some important research has been explored in this area in recent times. This Minireview emphasizes the conceptual approaches developed thus far, to tune and apply the hydricity parameter of transition-metal hydrides for efficient H2 evolution and CO2 reduction/hydrogenation catalysis focusing on the guiding principles for future research in this direction.