In contrast, Co (II) tetradentate complexes with Schiff base ligands offer enhanced structural rigidity and a more defined coordination geometry. The four donor atoms create a stable environment around the cobalt center, which can enhance the redox stability and catalytic turnover frequency . The tetradentate coordination also reduces the likelihood of ligand dissociation during the catalytic cycle, ensuring better durability and reusability of the complex. Moreover, cobalt's ability to undergo reversible oxidation states synergizes well with the Schiff base ligand, enabling efficient proton-coupled electron transfer mechanisms. As a result, Co(II) tetradentate complexes often exhibit superior HER performance compared to their Ni(II) bidentate counterparts, making them promising candidates for sustainable hydrogen production applications.