This manuscript describes a novel reactivity descriptor for Fe phthalocyanines molecular catalysts confined on the surface of OPG electrodes. The descriptor Delta E degrees'cat. is the gap in potential between the E degrees Fe(III)/(II) and E degrees Fe (II)/(I) redox processes for each FeN4 catalyst. This descriptor, labeled as the electrochemical hardness Delta E degrees'cat. Was examined using FeN4 macrocyclic complexes for the electrooxidation of 2-mercaptoethanol (2-ME) and thioglycolic acid TGA. Our findings reveal that the reactivity as log log(j/Gamma)E, plotted versus E degrees Fe(III)/(II) or E degrees Fe (II)/(I) give two linear correlations. The activity log(j/Gamma)E in both cases decreases with the redox potential of each FeN4 catalyst. In contrast log(j/Gamma)E, or log (TOF)E, increase with Delta E degrees'cat.. The Fe-S binding energy Delta E degrees'cat. Correlates linearly with Delta E degrees'cat. so the softer the system the greater the binding energy and the lower the catalytic activity.