We have investigated the size effect of the counter anion influencing to the mixed-valence state in MX chain platinum complexes. In order to study the counter-ion radius dependence of the mixed-valence state, we have measured the polarized reflectance spectra of [Pt(en) 2 ][PtCl 2 (en) 2 ]Y 4 (en=ethylenediamine, Y=PF 6 , HSO 4 , BF 4 ) in single crystals, and analyzed the crystal structure of [Pt(en) 2 ][PtCl 2 (en) 2 ](BF 4 ) 4 by X-ray diffraction method. From comparing the tetrafluoroborate, the perchlorate, the hydrogensulfate and the hexafluorophosphate in the series of the chloro-bridged complex, we found that the mixed-valence gap of platinum decreases with decreasing the counter-anion radius.