Infrared detection of the hysteresis in the thermally induced spin-crossover in bis(4,4 '-bis-1,2,4-triazole)bis(thiocyanato-N)iron(II) monohydrate

By the use of VT infrared spectroscopy, the thermally induced spin transiti on in the complex [Fe(btr)(2),(NCS)(2)]. H2O has been analyzed. The abbrevi ation btr stands for 4,4'-bis-1,2,4-triazole. This complex was reported ear lier to exhibit an abrupt spin transition with a large hysteresis effect. T he spin-transition curve calculated from the change in the absorption of th e CN-stretching vibrations of the thiocyanate ligand is similar to the curv e found by other techniques. The transition temperatures were found to be T (c)down arrow = 115 +/- 5 K and T(c)up arrow = 141 +/- 5 K, leading to a hy steresis of 26 K. These values for the same compound, deduced from magnetic and calorimetric measurements and Mossbauer spectroscopy are T(c)down arro w = 121 K and T(c)up arrow = 145 K. The spin-transition curve was analyzed thermodynamically, leading to the following parameters: Delta H= 9.7 kJ mol (-1), Delta S = 79 J mol(-1) K-1 and Gamma = 3.15 kJ mol(-1) in agreement w ith earlier determined values. (C) 2000 Elsevier Science S.A. All rights re served.