5T2-1A1 Spin transition and residual paramagnetism in bis(2,4-bis(2-pyridyl)thiazole)iron(II) complexes: mössbauer effect and magnetism

The ⁵⁷Fe Mössbauer effect in Fe(pythiaz)₂] (BF₄)₂ (I) and Fe(pythiaz)₂] (C&O₄)₂ (II) has been studied between 298 and 4.2°K (pythiaz = 2,4-bis(2-pyridyl)thiazole). At 298°K compound I shows a doublet with ΔE_Q(⁵T₂) = 1.29 mm sec^?1 and δ^1S(⁵T₂) = +0.93 mm sec^?1 characteristic of a ⁵T₂ ground state. At 236°K, a second doublet, typical for a ¹A₁ ground state appears. The transition ⁵T₂ å ¹A₁ progresses as the temperature is lowered but levels off below ≈ 120°K. At 4.2°K, 59% of the intensity is due to the ¹A₁ state, and ΔE_Q(¹A₁) = 1.59 mm sec^?1 and δ^1S(¹A₁) = +0.26 mm sec^?1. In an applied magnetic field, V_zz(¹A₁) < 0 has been determined Similar results have been obtained with compound II.Debye-Waller factors f5_T2 and f1_A1. were determined from the Mössbauer spectra under the assumption of Curie-Weiss dependence of the magnetism for the ⁵T₂ and constant μ_eff for the ¹A₁ ground state. The resulting temperature dependence of f1_A1 is highly unusual thus suggesting complicated magnetic behaviour of both ground states in the transition region. Two mechanisms for the nature of the transition are discussed, a “spin-flip” mechanism being the physically more reasonable one. The assumption of a simple Boltzmann distribution (“spin equilibrium”) may be ruled out for the solid but could be encountered in solutions.