Carbon cluster (C(c)) substituents have been shown to be of essential importance in C(c).C(c) distance, rotational energy barriers, and (11)B[(1)H] NMR chemical shift values in mixed pyrrolyl/dicarbollide cobalt complexes. In the present work, the influence of electronic properties of exo-cluster substituents upon redox potential values associated to the metallic central atom in mixed pyrrolyl/dicarbollide and dimethylpyrrolyl/dicarbollide cobalt complexes is discussed. With that purpose, two new neutral sandwich species, closo-[3-Co(eta(5)-NC(4)(CH(3))(2)H(2))-1,2-(C(6)H(5))(2)-1,2-C(2)B(9)H(9)] (2) and closo-[3-Co(eta(5)-NC(4)(CH(3))(2)H(2))-1-CH(3)-2-SCH(3)-1,2-C(2)B(9)H(9)] (3), have been synthesized and characterized by (1)H, (11)B, (11)B[(1)H], and (13)C[(1)H] NMR and IR spectroscopies, elemental analysis, and X-ray diffraction analysis. The redox potential (E(1/2)) of these complexes has been measured in nonpolar media and compared to values obtained for previously reported mixed complexes, incorporating alkyl, phenyl, thiophenyl, and thiomethyl exo-cluster substituents. The potential shift arising from the effect of these substituents has been discussed in terms of individual and average contribution. This last point is in the case of two identical substituting groups placed on both C(c) atoms, in which the contribution of the second introduced substituent has shown to be lower than that for the first one. The potential shift arising from the presence of methyl units on the pyrrolyl anion has also been determined.