According to the theory of additive interaction energy in mixed-valence polynuclear complexes, a trinuclear ring complex composed of the reduced center R and the oxidized center O should show three voltammetric waves with a common potential separation. The common separation is inconsistent with experimental results. A quantum chemical approach was made, in which a linear combination of redox wavefunctions for the R and O gave the interaction energies of mixed valence states of ROR and ORO as a function of the overlap integral S between R and 0, and pair interaction energies of O-O, R-R and R-O. The approach solved the following frustration : if one R in the ROR were to donate partially the electron to the O to form the stable R-O pair, the other R might suffer from the unsuccessful donation; conversely, if either 0 of the ORO could accept partially the electron from the R, the other O might remain vacant. The resonance in the ring was thought to interfere with the formation of such a strong R-O pair as in a dinuclear complex. The interaction energies by the quantum approach provided three standard potentials, which predicted three voltammetric peak potentials. Whether the potential of the second wave was more positive or negative than the middle potential of the first and third waves depended on S and the difference between the two pair interaction energies of R-R and O-O. This model explained consistently the relation between the negative potential shift of the second wave and exhibition of the IR absorption of RRR in [CpCo(S2C2)](3).