We have investigated mid-gap absorption bands, their photo-induced absorption effect at various temperatures between liquid helium temperature and room temperature, and crystal structures between 150 K and 300 K in a neutral MX chain system, [PtX(2)(en)][PtX(4)(en)] (X=Cl, Br, I), which have one-dimensional structure, ...Pt-II...X-Pt-IV-X.... Temperature dependence of the intensity of the mid-gap absorption bands and the growth of the photo-induced absorption in [PtCl2(en)][PtCl4(en)] show that the generation efficiency of the mismatch of valence-alternation Is changed at 200-220 K. Temperature dependence of lattice parameters of [PtCl2(en)][PtCl4(en)] shows that the structural phase transition occurs at 220 K from the orthorhombic system to the monoclinic system by cooling. Comparison of the crystal structure between 195 K and 296 K shows that the basic structure does not change at the phase transition. The conformation of the chelate ring of the ethylenediamine is changed from the disordered state to the ordered state at the transition point by cooling. This result suggests that the order-disorder transition of the conformation of the ethylenediamine would be responsible for the generation efficiency of the mismatch of the valence alternation.