Phase transformations in a double complex salt of the ruthenium nitrosyl anion and tetraamine-palladium cation

Abstract

Second-order phase transformation in double complex salt (DCS) [Pd(NH₃)₄][RuNO(NO₂)₄OH] was examined by DSC, single-crystal diffraction and powder diffraction. All data confirm the reversible “order–disorder” phase transition without an experimentally detected sharp volume change in the temperature range of 343–363 K. At low temperature, the less symmetric triclinic phase (P) is stable, while the monoclinic phase (P2/m) is stable at high temperature. The main difference between crystal structures is the disorder of the ruthenium anion in the monoclinic phase due to the presence of the ruthenium atom in a special position. The additional symmetry plane in the P2/m phase includes ON–Ru–O atoms, resulting in the rotational disorder of NO₂ groups. The entropy of transformation determined by differential scanning calorimetry corresponds to order–disorder transformation with a 3-fold increase in microstate number.