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

Abstract

A phenomenological thermodynamic model suited for the interpretation of the high spin⇌low spin transition in solidtransition 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 polycrystallinesolid solutions of [Fe x Zn1−x (2‐pic)3]Cl2⋅C2H5OH(2‐pic=2‐picolylamine) with 1.0⩾x⩾0.0009. The spin transition temperature T c 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 T c (x) as well as for the effective enthalpy and entropy changes ΔH eff(x) and ΔS eff(x) have been adjusted to the measured data. The average domain size for the pure iron compound has been calculated to be n≈3.5 which indicates comparatively weak intermolecular coupling strength for the present system. The domain size n depends markedly on the iron concentration x and tends to one at very low iron concentration. The volume expansion of the domain size n approximated by n (x) =1+n o x γ, decreases rather strongly with falling iron concentration (γ≈1.7). In the pure iron compond, the sum of the intramolecular enthalpy contributions changes by some 550 cal mol−1 and that of the intermolecular enthalpy contributions, responsible for the cooperative effect, by some 200 cal mol−1 on going from low spin to high spin.