Reduction potentials (E-0) of six mononuclear cobalt catalysts (1-6) for hydrogen evolution reaction and electron donating/withdrawing effect of nine X-substituents on their macro cyclic ligand are reported at solvation effect-included B3P86/6-311+G** level of density functional theory. The electrostatic potential at the Co nucleus (V-Co) is found to be a powerful descriptor of the electronic effect experienced by Co from the ligand environment. The V-Co values vary substantially with respect to the nature of macrocycle, type of apical ligands, nature of substituent and oxidation state of the metal center. Most importantly, V-Co values of both the oxidized and reduced states of all the six complexes show strong linear correlation with E-0. The correlation plots between V-Co and E-0 provide an easy-to-interpret graphical interpretation and quantification of the effect of ligand environment on the reduction potential. Further, on the basis of a correlation between the relative V-Co and relative E-0 values of a catalyst with respect to the CF3-substituted reference system, the E-0 of any X-substituted 1-6 complexes is predicted.