Interpretation of high spin⇌low spin transition in iron (II) complexes. I. A phenomenological thermodynamic model

A phenomenological thermodynamic model suited for the interpretation of the high spin⇌low spin transition in solid transition metal complexes is described. The model is based on the idea of the spin transition taking place through a significant phonon‐assisted coupling between the electronic state of the primary spin change center and n−1 neighboring complex molecules undergoing secondary spin transitions and thus forming a cooperative domain as suggested by Sorai and Seki. The model has been applied to interpret the spin transition in polycrystalline solid solutions of [FexZn1−x(2‐pic)3]Cl2⋅C2H5OH(2‐pic=2‐picolylamine) with 1.0⩾x⩾0.0009. The spin transition temperature Tc may be calculated using the effective changes of enthalpy and entropy, respectively, obtained from 1nK vs 1/T plots. Phenomenological expressions for the transition temperature Tc (x) as well as for the effective enthalpy and entropy changes ΔHeff(x) and ΔSeff(x) have been adjusted to the measured data. The average domain size for the p...