**1.3 Metal and redox-active ligands for HER**

immersed solar energy and triggered the electron transfer reaction. The most important criteria for the solar-driven water-splitting reaction are electronic band gap matching of the photosensitive material to the redox potential of water [39]. Metal complexes act as chromophore associated with mainly three types of electron transfer: metal center (MC), ligand center (LC), and metal–ligand center transition (MLCT). The MLCT state of the metal complex plays a crucial role in photocatalytic reactions. In octahedral complexes with conjugated ligand system, the highest occupied molecular orbital (HOMO) corresponds to the metal-localized t2g-orbitals, and the lowest unoccupied molecular orbital (LUMO) is associated with antibonding π\*-orbital localized on the ligands. On the absorption of UV–visible light, an electron is promoted from one of the metal-centered t2g orbitals to a ligandcentered π\* orbital, resulting in the MLCT state shown in **Figure 2(b)**. As a result, the redox properties of the metal complexes are dramatically changed. The excited metal complexes behave as better oxidants and better reductants than their electronic ground state and can hold more thermodynamic driving force for the charge transfer reactions. Based on the photo-induced redox potential changes and the long-lived lifetime of the excited state, many metal complexes have been intensively investigated as chromophores for this photocatalytic H2 production purpose [40]. Zou and coworkers have described various photocatalytic systems for H2 production, which exposed that most of the photocatalytic systems suffer photodecomposition and instability [40]. Hence, for long-term use, it is imperative to build up highly proficient H2 generation systems with long lifetimes and high durability.

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

Many reviews have been published on solar H2 evolution systems based on

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

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

photocatalysts [41–44].

**1.2 Mechanistic pathway**

**Figure 3.**

**100**

*intermediate.*

Here we start by describing the fundamental concept of metal and organic ligand system which gives a strong influence on the performance of H2 evolution. Transition metal cations with partial filled d-electronic configurations are considered as catalyst. The characteristic feature of this type of catalyst is that the metal ions can exist in higher oxidation state [44]. There are several literatures reported with partially filled d-orbital which show high stability toward water-splitting reactions [50–53]. However, the most catalytic system requires very high temperature and precious metal at the active site; therefore, it will be a challenge for researchers to develop a photocatalytic and electrocatalytic system at low temperature with lowcost metal [54–57]. Indeed, first-row transition metal complexes (Co, Ni, Cu, Zn) have been exploited in the last few decades for this purpose. Beyond the reactivity of metal, the redox activity of organic ligands has also received continuous attention. The redox-active ligand works as electron sink in the complexes and maintains the metal in its original oxidation state. Redox-active ligands convey a novel reactivity to the complex by loss or gain of electrons [58]. In addition, the redox-active property of the ligand can also be influenced by the modification of the substituents by σ and π donating ability, π accepting ability, and conjugation [59]. A highly conjugated system such as bpy, porphyries, and ortho-phenylenediamine (opda) having anti-bonding π\*-orbital localized on the ligands is considered for hydrogen production due to its multielectron or multiproton pooling ability which is responsible for dramatically changing the potential of redox properties [60, 61]. Substituents attached to redox-active ligands, electron density, and charge on the metal ions also effect the standard electrode potential.

So far, considerable advancement has been done in the field of electrocatalytic and photocatalytic water-splitting reaction for hydrogen production, and several advance review papers have been reported by scientists [40, 51, 62–64]. However, very limited comprehensive tutorial has published on only first-row transition metal-based catalysts. This chapter describes electrocatalytic as well photocatalytic properties of inorganic catalysts and their structural and mechanistic features. Here we put an effort elucidate the direction of fundamental mechanistic aspects during electrocatalytic and photocatalytic hydrogen (H2) production reaction (HER).
