A series of trans-heteroleptic paddlewheel- type diruthenium(II,II) complexes [Ru-2(2,6-(CF3)(2)PhCO2)(2)(p-RPhCO2)(2)(THF)(2)] was synthesized by substituting para-R-substituted benzoate ligands (p-RPhCO2-, R = CF3, Cl, F, H, Ph, Me, MeO, OH) in [Ru-2(p-RPhCO2)(4)(THF)(2)] with 2,6-bis(trifluoromethyl)-benzoate (2,6-(CF3)(2)PhCO2-). The redox potential (E-1/2) for the [Ru-2(II,II)]/[Ru-2(II,III)](+) couple and the HOMO energy level were linearly dependent on the acidity (pK(a)) of the substituted benzoic acids (p-RPhCO2H) and Hammett constants of p-R substitution groups.