Observation of charge bistability in quasi-one-dimensional halogen-bridged palladium complexes by X-ray absorption spectroscopy.

We performed X-ray absorption fine structure (XAFS) measurements on three representative bromo-bridged palladium compounds. In the X-ray absorption near-edge structure (XANES) spectra, the averaged-valence (AV) compound, [Pd3+(dabdOH)2Br]Br2 (1: dabdOH = (2S,3S)-2,3-diaminobutane-1,4-diol), and the mixed-valence (MV) compound, [Pd2+(en)2]Pd4+(en)2Br24 (2), showed significant differences in their spectra. In Pd(en)2Br2·H2O (3: en = ethylenediamine, Suc-C5 = dipentylsulfosuccinate), which exhibits an MV-AV phase transition, on the other hand, the spectroscopic difference between below and above the phase transition temperature was hardly observed due to the subtle difference in the oxidation states. In the extended X-ray absorption fine structure (EXAFS) spectra, a clear difference in the Pd-Br correlation region was observed upon the phase transition.