Understanding spin interactions in transition metal complexes is fundamental to designing and optimizing electrocatalysts. In particular, spin interactions between the metal center and ligands significantly influence the electronic structure, magnetic properties, and reactivity of these complexes, ultimately affecting catalytic efficiency. In this study, we investigate the role of spin interactions in the electrocatalytic hydrogen evolution reaction (HER), focusing on their impact on electron transfer processes. We reveal that exchange coupling interactions in high-spin states vary with the coordination environment and directly modulate HER activity. Additionally, we demonstrate that the influence of the metal center on electron transfer differs between high- and low-spin states. Moreover, our study explores how the pKa of Brønsted acids and spin-state variations alter the proton-coupled electron transfer (PCET) mechanism. These findings provide new insights into the relationship between spin states and catalytic performance, offering valuable guidelines for the rational design of next-generation transition metal-based electrocatalysts. In this presentation, we will discuss our recent results and their implications for catalyst development.