Metal ion complexes of semiquinone radical anions exhibit different types of thermochromism depending on metal ions and quinones. Metal ion complexes of 1,10-phenanthroline-5,6-dione radical anion (PTQ(center dot-))produced by the electron-transfer reduction of PTQ by 1,1'-dimethylfer-rocene (Me(2)Fc) in the presence of metal ions (Mg2+ and Sc3+) exhibit the color change depending on temperature, accompanied by the concomitant change in the ESR signal intensity. In the case of Mg2+, electron transfer from Me(2)Fc to PTQ is in equilibrium, when the concentration of the PTQ(center dot-)-Mg2+ complex (lambda(max) = 486 nm) increases with increasing temperature because of the positive enthalpy for the electron-transfer equilibrium. In contrast to the case of Mg2+, electron transfer from Me(2)Fc to PTQ is complete in the presence of Sc3+, which is a much stronger Lewis acid than Mg2+, to produce the PTQ(center dot-)--Sc3+ complex (lambda(max)= 631 nm). This complex is in disproportionation equilibrium and the concentration of the PTQ(center dot-)-Sc3+ complex increases with decreasing temperature because of the negative enthalpy for the proportionation direction, resulting in the remarkable color change in the visible region. On the other hand, the p-benzosemiquinone radical anion (Q(center dot-)) forms a 2:2 pi-dimer radical anion complex [Q(center dot-)-(Sc3+)(2)-Q] with Q and Sc3+ ions at 298 K (yellow color), which is converted to a 2:3 pi-dimer radical anion complex [Q(center dot-)-(Sc3+)(3)-Q] with a strong absorption band at lambda(max) = 604 nm (blue color) when the temperature is lowered to 203 K. The change in the number of binding Sc3+ ions depending on temperature also results in the remarkable color change, associated with the change in the ESR spectra.