The titlePt compound is a highly-conductive mixed-valence MMX-chain system. It exhibits a metal-semiconductor transition at 300 K, which is attributed to a valence transition of Pt from an average-valence state of +2.5 to a trapped-valence state of +2 and +3. From I-129 Mossbauer spectroscopic study, the 1-D structural mode was strongly suggested to be a -Pt-II -Pt-III-I-Pt-III-Pt-II -I-Pt-II-Pt-III-I-Pt-III-Pt-II- charge ordering state at 16 K. The charge-ordering mixed-valence state is discussed, as well as the valence state of the Ni compound, which is considered to be an averaged-valence Mott insulator.