The optical gap in a series of bromo-bridged platinum chain complexes, [Pt(en)2Br](C(n)-Y)2·H2O (en = ethylenediamine; C(n)-Y = dialkyl sulfosuccinate; n = the number of carbon atoms), was controlled by using chemical pressure. From the single-crystal structure, [Pt(en)2Br](C6-Y)2·H2O is in a mixed-valence state at 200 K. In addition, Pt-Pt distances decreased with an increase in n or with a decrease in the temperature. Continuous decreases in the optical gaps upon cooling were observed for n = 5, 7. The smallest gap of 1.20 eV was observed for n = 7 at 50 K. For n = 12, the complex was still in a mixed-valence state at 5 K, although the Pt-Pt distance was quite short. This is probably because of the energetic mismatch between 5dz(2) orbitals of the Pt ions and 4pz orbitals of the Br ions.