Discrete cyanide-bridged Co-Fe multinuclear complexes can be considered as functional units of bulk Co-Fe Prussian blue analogues, and they have been recognized as a new class of switching molecules in the last decade. The switching property of the cyanide-bridged Co-Fe complexes is based on intramolecular electron transfers between Co and Fe ions, and we herein refer to this phenomenon as an electron transfer-coupled spin transition (ETCST). Although there have been numerous reports on the complexes exhibiting ETCST behavior, the systematic study of the substituent effects on the thermal ETCST equilibrium in solution has not been reported yet, and the rational control of the equilibrium temperature remains challenging. We report here the syntheses and thermal ETCST behavior both in the solid state and solution for a series of tetranuclear [Co₂Fe₂] complexes, [Co₂Fe₂(CN)₆(L1)₂(L2)₄]X₂ (L1 and L2: tri- and bidentate capping ligands for Fe and Co ions, X: counteranions). All complexes showed thermal ETCST equilibrium between high-spin ([(hs-Co^II)₂(ls-Fe^III)₂]) and low-spin ([(ls-Co^III)₂(ls-Fe^II)₂]) states in butyronitrile, and the equilibrium temperatures (T_1/2) showed systematic shifts by chemical modifications and chemical stimuli. The T_1/2 values were correlated with the redox potential differences (ΔE) of the Fe and Co ions in the constituent units, and the larger ΔE values led to the lower T_1/2. The present result suggests that the thermal ETCST behavior in solution can be rationally designed by considering the redox potentials of the constituent molecules.

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