The superior electron mobility of the Ni (II) complex, driven by its uniformly negative band energy differences, provides a more efficient charge transport mechanism. This enhances its catalytic efficiency by ensuring a steady and rapid supply of electrons for the reduction process. On the other hand, the Co (II) complex, with its varied band energy profile, may suffer from slower electron transfer rates and increased recombination losses, limiting its effectiveness in HER. Hence, the Ni (II) complex's electronic structure not only supports better electron mobility but also positions it as a more efficient catalyst for HER.