In the case of iron mixed-valence complexes whose spin states are situated in the spin-crossover region, conjugated phenomena coupled with spin and charge are expected. In general, the Fe site coordinated by six S atoms is in the low-spin state, while the Fe site coordinated by six O atoms is in the high-spin state. From this viewpoint, we have synthesized and investigated physical properties for an monothiooxalato-bridged (mto = C 2 O 3 S) iron mixed-valence complex, (n-C 4 H 9 ) 4 N[Fe II Fe III (mto) 3 ], consisting of Fe III O 3 S 3 and Fe II O 6 octahedra, which behaves as a ferrimagnet with its magnetic transition temperature of T N = 38 K and Weiss temperature of θ = -93 K. From the analysis of 57 Fe Mössbauer spectra of 57 Fe enriched complexes, (n-C 4 H 9 ) 4 N[ 57 Fe II Fe III (mto) 3 ] and (n-C 4 H 9 ) 4 N[Fe II57 Fe III (mto) 3 ], the charge transfer between Fe II and Fe III exists in the paramagnetic phase. Considering the time window of 57 Fe Mössbauer spectroscopy, the time scale of the valence fluctuation is at least slower than 10 -7 s. In order to confirm the valence fluctuation between Fe II and Fe III , we investigated the dielectric constant and found an anomalous enhancement attributed to the Fe valence fluctuation between 170 and 250 K.
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