NATURE OF THE SPIN-STATE TRANSITIONS IN IRON(II) 2,2'-BI-2-THIAZOLINE COMPLEXES BASED ON HEAT-CAPACITY MEASUREMENTS

On the basis of heat capacity measurements by the DSC technique, it is demonstrated that the spin-state transitions in [Fe(bt)2(NCS)2] and [Fe(bt)2(NCSe)2] are thermodynamically first order (bt = 2,2'-bi-2-thiazoline). Values of various thermodynamic parameters are determined. In particular, DELTA-H = 9.54 kJ mol-1 and DELTA-S = 54.4 J K-1 mol-1 for [Fe(bt)2(NCS)2] and DELTA-H = 9.11 kJ mol-1 and DELTA-S = 41.5 JK-1 mol-1 for [Fe(bt)2(NCS)2]. For [Fe(bts)2(NCS)2], a broad maximum of C(p) is observed (bts = 2,2'-bi-5-methyl-2-thiazoline). The spin-state transition may thus be considered as thermodynamically higher order, the resulting values of the enthalpy and entropy of the transition being DELTA-H congruent-to 5.7 kJ mol-1 and DELTA-S congruent-to 27 JK-1 mol-1, respectively. Within the Frenkel theory of heterophase fluctuation, the results demand a domain size between 51 and 116 molecules for the first-order transition compounds, whereas for [Fe(bts)2(NCS)2] domains of only approximately 12 molecules are obtained. Within an Ising model for interacting HS and LS molecules, the results infer a stronger interaction for first-order transitions than for the continuous transition in [Fe(bts)2(NCS)2]. Apparently, the weak interaction between small domains in [Fe(bts)2(NCS)2] produces a large fluctuation of HS and LS molecules which gives rise to the continuous appearance of the spin-state transition.