**Abstract**

The design and modification of metal–organic complexes for hydrogen (H2) gas production by water splitting have been intensively investigated over the recent decades. In most reported mechanistic pathways, metal hydride species are considered as crucial intermediates for H2 formation where the metal present at the active site plays an imperative role in the transfer of electron and proton. In the last few decades, much consideration has been done on the development of non-precious metal–organic catalysts that use solar energy to split water into hydrogen (H2) and oxygen (O2) as alternative fossil fuels. This review discussed the design, fabrication, and evaluation of the catalysts for electrocatalytic and photocatalytic hydrogen production. Mechanistic approach is addressed here in order to understand the fundamental design principle and structural properties relationship of electrocatalysts and photocatalysts. Finally, we discuss some challenges and opportunities of research in the near future in this promising area.

**Keywords:** redox-active-ligand, first-row transition metals, hydrogen evolution, catalytic cycle, electrocatalyst, photocatalyst
