CALORIMETRIC STUDY ON THE SPIN-CROSSOVER PHENOMENON BETWEEN (3)T(1) AND (5)E IN TRANS-BIS[1,2-BIS(DIETHYLPHOSPHINO)ETHANE]DIIODOCHROMIUM(II), [CRI2(DEPE)2]

The molar heat capacity of [CrI2(depe)2], where depe = trans-bis[1,2-bis(diethylphosphino)ethane], has been measured with an adiabatic calorimeter in the 14-300 K range, and infrared spectra have been recorded at 300 and 91 K in the 4000 30 cm -1 range. A sharp heat capacity anomaly arising from the spin-state transition from the low-spin 3T1 to the high-spin 5E electronic ground state was found at 171.45 K. The enthalpy and entropy of the phase transition are DELTA(trs)H = 6686 +/- 33 J mol-1 and DELTA(trs)S = 39.45 +/- 0.22 JK -1 mol -1, respectively. The entropy gain can be well accounted for in terms of the contributions from the change in the spin manifold (10%), the change in the metal-ligand skeletal vibrations (approximately 75%) and the change in the barrier heights hindering the internal rotation of the eight methyl moieties of the ethyl groups attached to the phosphine ligands (approximately 15%). The extremely sharp phase transition observed for this complex has been explained in terms of the large polarizability and thus large compressibility of the iodide ligands. The ''cooperativity'' often used for spin-crossover phenomena is discussed.