Iron mixed-valence complex, (n-C3H7)4N[FeIIFeIII(dto)3] (dto =C2O2S2), shows a spin-entropy driven phase transition called charge transferphase transition around 120K and a ferromagnetic transition at 6.5 K. These phase transitions remarkably depend on the hexagonal ring size intwo-dimensional honeycomb network structure of [FeIIFeIII(dto)3]∞. In order to c...