**1.2 Mechanistic pathway**

So far, extensive theoretical study has been revealed, the possible mechanistic process of proton reduction to hydrogen evolution through transition metal molecular catalyst. A generalized mechanistic scheme depicting the homolytic and heterolytic path is shown in **Figure 3**. The homolytic mechanism involves bimetallic route, where a metal hydride species ([Mn+–H]) react with another metal hydride to release one H2 via reductive elimination. Instead, heterolytic is a monometallic

**Figure 3.** *Proposed homolytic and heterolytic mechanisms for H2 evolution via the formation of a metal hydride (M-H) intermediate.*

*Recent Progress of Electrocatalysts and Photocatalysts Bearing First Row Transition Metal… DOI: http://dx.doi.org/10.5772/intechopen.92854*

pathway, where the metal hydride [Mn+–H] is further reduce and protonated for H2 evolution [45]. Both pathways function simultaneously, two protons and two electrons are delivered to the metal center, and in few cases, the pH, catalytic concentration, and proton source decide the dominant route [46]. During the past decade, a number of review articles emphases on the structural property relationship and mechanistic study [45, 47–49]. Among all research on catalyzed H2 evolution, the mechanistic investigation on proton reduction catalysis is essential because it can give us a significant idea to design better molecular catalysts in the future [49].
