The charge-ordering states with lattice distortions of a halogen-bridged binuclear-metal mixed-valence complex (called MMX chain), Pt 2 ( dta ) 4 I ( dta = CH 3 CS 2 - ), have been investigated by transport, magnetic, and optical measurements. This complex is a binuclear unit-assembled conductor containing metal−metal bonds. It exhibits metallic conduction above room temperature, representing the first example of a metallic halogen-bridged one-dimensional transition-metal complex. Below 300 K it shows semiconducting behavior, which is considered to be of the Mott−Hubbard type due to electron correlation. The metal−semiconductor transition at 300 K (= T M - S ) is derived from a valence transition of Pt from an averaged-valence state of 2.5+ to a trapped-valence state of 2+ and 3+. The charge-ordering modes are considered to be −I Pt 2+ −Pt 3+ −I Pt 2+ −Pt 3+ −I Pt 2+ −Pt 3+ −I Pt 2+ −Pt 3+ −I for the semiconducting phase below T M - S and −I−Pt 2.5+ −Pt 2.5+ −I−Pt 2.5+ −Pt 2.5+ −I−Pt 2.5+ −Pt 2.5+ −I−Pt 2.5+ −Pt 2.5+ −I− for the metallic phase above T M - S . 129 I Mössbauer spectroscopic study is reported for a low-temperature insulating phase below 80 K. The low-temperature electronic structure is considered to be an alternate charge-ordering state with lattice distortions of I Pt 2+ −Pt 3+ −I−Pt 3+ −Pt 2+ I Pt 2+ −Pt 3+ −I−Pt 3+ −Pt 2+ I . The present binuclear platinum complex inherently possesses valence instability of the intermediate valence 2.5+. X-ray photoelectron spectroscopy and polarized reflection measurements are also reported.
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Kitagawa et al. (1999) studied this question.
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