CALORIMETRIC STUDY OF THERMOCHROMIC COMPLEXES. 2. HEAT CAPACITY AND PHASE TRANSITION OF BIS(<i>N,N</i>‐DIETHYLETHYLENEDIAMINE)COPPER(II) PERCHLORATE

Abstract

Abstract The thermochromic phase transition of bis(N,N-diethylethylenediamine) copper(II) perchlorate has been studied by adiabatic calorimetry in the 12–359 K temperature range. A large heat capacity anomaly was observed at 317.64 K with a long heat-capacity tail extending down to ∼200 K. The enthalpy and entropy of the phase transition were found to be δtrs H = 17.43 kJ mol−1 and δtrs S = 55.21 J K−1 mol−1, respectively. Together with a calorimetric study of a homologous thermochromic complex, bis(N,N-diethylethylenediamine)copper(II) tetrafluoroborate (J. Phys. Chem. Solids, 55, 99 (1994)), the nature of the present thermochromic phase transition is well described by a puckering motion of the copper-ligand chelate rings and a change in the ligand-field strength estimated on the basis of the angular overlap model. The correlation between structures, electronic spectra and thermal properties is discussed.