Calorimetric study of the halogen-bridged mixed-valence binuclear metal chain complex ${\mathrm{Pt}}_2(n-Bu{\mathrm{CS}}_2{)}_4\mathrm{I}$ $(Bu$=butyl chain)

Heat capacity of a halogen-bridged quasi-one-dimensional mixed-valence binuclear metal complex (the so-called $\mathrm{MMX}$ chain), ${\mathrm{Pt}}_2(n-\mathrm{Bu}{\mathrm{CS}}_2{)}_4\mathrm{I},$ was measured by adiabatic calorimetry. First-order phase transitions were observed at 213.5 K and 323.5 K. For the former, the enthalpy and entropy of transition were $4.29{\mathrm{kJ}\mathrm{}\mathrm{mol}}^{-1}$ and $20.09{\mathrm{J}\mathrm{}\mathrm{K}}^{-1}{\mathrm{mol}}^{-1},$ respectively. Those of the latter were $2.41{\mathrm{kJ}\mathrm{}\mathrm{mol}}^{-1}$ and $7.46{\mathrm{J}\mathrm{}\mathrm{K}}^{-1}{\mathrm{mol}}^{-1},$ respectively. Another thermal anomaly probably due to a higher-order phase transition was detected at 114 K. The magnitude of the entropy of transition shows that, upon heating, the butyl chains in one-third complexes in crystalline ${\mathrm{Pt}}_2(n-\mathrm{Bu}{\mathrm{CS}}_2{)}_4\mathrm{I}$ are changed from an ordered state to a disordered state through the phase transition at 213.5 K, and resume the ordered state from this disordered state at 323.5 K. The transition at 213.5 K involves a “spin-Peierls” contribution beyond the structural one.