Phase transitions in iron(II) hexafluorosilicate and perrhenate complexes with 4-amino-1,2,4-triazole

The heat capacity and magnetic susceptibility of iron(II) hexafluorosilicate and perrhenate coordination compounds with 4-amino-1,2, 4-triazole (A), FeA(3)SiF(6) . H2O (1) and FeA(3)(ReO4)(2) (II), were measured in the temperature range 80-350 K. The (1)A(1) <----> T-5(2) spin transition was observed in I and II. The magnetochemical data were used to determine the transition temperatures during heating (255 and 228 K for I and 11, respectively). Heat capacity anomalies with maxima at 251 K for I and 231 K for II well correlated with magnetic susceptibility measurements. The enthalpies and entropies of spin transitions were determined for both compounds. One more anomaly with a maximum at 323 K was observed for I. This anomaly might be caused by a change in the structure of the anion during heating.