Study of the linkage isomerization and its influence on the charge transfer phase transition for iron mixed-valence complexes by means of Moessbauer spectroscopy

In the mixed-valence complex where the ligand field exists in the spin crossover region, a new type of phase transition phenomenon where spin and charge are interlocked so that the free energy of the whole system becomes most stable is seen. The authors investigated this phenomenon. As temperature decreases from room temperature in an iron mixed-valence complex showing ferromagnetism, electrons collectively move from Fe^II to Fe^III despite the fact that this material is an insulator. The authors clarified this charge-transfer phase transition using Moessbauer spectroscopy. It is conceivable that dto (=C₂O₂S₂) inverts in the reaction solution and partially causes linkage isomerism. Using pure iron in which ⁵⁷Fe is concentrated to 96% as a starting material, they synthesized the complexes in which Fe^II site was replaced by ⁵⁷Fe coordinated with oxygen atoms and the complexes in which Fe^III site was replaced by ⁵⁷Fe coordinated with sulfur atoms, and quantitatively investigated conditions under which linkage isomerization occurs. As a result, it was revealed that dto of the crosslinking ligand inverts by 180 degree in a solution and partially causes linkage isomerism (Fe^III-S₂C₂O₂-Fe^II → Fe^III-O₂C₂S₂-Fe^II). In addition, when the complexes precipitate in the state of occurrence of linkage isomerism (Fe^III-O₂C₂S₂-Fe^II), the states of Fe^IIS₆(S=0) and Fe^IIIS₆(S=5/2) are formed. It was revealed that these states are only stable at 120 K or under, and charge transfer occurs in solid state forming Fe^IIIS₆(S=1/2) and Fe^IIS₆(S=2). (A.O.)