The charge-ordering states with lattice distortions of a halogen-bridged binuclear-metal mixed-valence complex (called MMX chain), Pt-2(dra)(4)I (dta = CH3CS2-), 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 (= TM-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-Pt2+-Pt3+-I-Pt2+-Pt3+-I-Pt2+-Pt3+-I-Pt2+-Pt3+-I-for the semiconducting phase below TM-S and -I-Pt2.5+-Pt2.5+-I-Pt2.5+-Pt2.5+-I-Pt2.5+-I-Pt2.5+-Pt2.5+-I-for the metallic phase above TM-S. I-129 Mossbauer 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-Pt2+-Pt3+-I-Pt3+-Pt2+-I-Pt2+-Pt3+-I-Pt3+-Pt2+-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.