**Author details**

Ni-, Cu-, Zn-based complexes for HER. We have recapitulated the fundamental principles of hydrogen and oxygen evolution reactions with molecular complexes. The designing and fabrication of the molecular complexes with redox-active ligands have been discussed in details; HER activity of the complexes strongly dependent on redox-active ligands as well the central metal ions are discussed in detail. A mechanistic approach and transfer of electron and proton during the homogeneous electrocatalyst and photocatalysts cycle are given in point. Although reasonable

electrocatalysts and chromospheres for photocatalytic hydrogen production, still several issues exist which need further improvement: (i) some photocatalytic systems suffer from low activities and short life times which is manifested in the instability of catalytic systems and so concern on the systems with modest water splitting activity and poor stability of the complexes. (ii) most of the complexes are not soluble in water leading to the use of organic solvent or mixture of organicwater solvent. From the future prospective, it is required to develop redox-active ligands with substituted functional group to increase the solubility of complexes in water. More experimental, spectroscopic, magnetic, and theoretical investigations is still needed to be carried out in order to understand the ligand- and metal-centered electron transfer processes. (iii) In addition, the overpotential requirements for most of the organic ligands are still very high, a chelating ligands giving much lower thermodynamic potentials and much smaller oxidation potential that should be utilized in future. (iv) In the regard of future growth in this field, with the need to design molecular complexes that can be immobilized on the surface of the electrode, for this purpose addition of suitable functional group in the ligand is necessity. These complexes can also be supported by the development of surface of the solid photocatalyst, like TiO2, BiVO4, etc. to demonstrate efficient photoelectro-

The authors acknowledges the helps received from Ms. Priyadarshini Sahu and

Mr. Abhineet Verma during the manuscript preparation.

progress has been made in the development of metal complexes based

*Photophysics, Photochemical and Substitution Reactions - Recent Advances*

chemical cell.

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**Acknowledgements**

Shipra Sagar<sup>1</sup> , Ravi K. Kanaparthi<sup>2</sup> , Manish K. Tiwari<sup>3</sup> and Satyen Saha<sup>3</sup> \*

1 Department of Chemistry, Central University of Jharkhand, Ranchi, India

2 Department of Chemistry, Central University of Kerala, Kerala, India

3 Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi, India

\*Address all correspondence to: satyen.saha@gmail.com

© 2020 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
