Thio derivatives of β-diketones and their metal chelates. X. The anomalous paramagnetism of some iron(III) chelates of monothio-β-diketones

Abstract

The magnetic susceptibilities of the iron(111) chelates Fe(R¹CS=CHCOR²)₃ (R¹ = Ph, p-MeC₆H₄, p-MeOC₆H₄, p-BrC₆H₄, 2-thienyl, 2-furyl, R² = CF₃; R¹ = Ph, R² = Ph, OEt) have been investigated over a temperature range. The iron(111) complex of ethyl thiobenzoylacetate (R¹ = Ph, R² = OEt) is of the high-spin type and obeys the Curie-Weiss law with θ =-8ºk. The moments of the other complexes vary between 2.31 and 6.61 B.M. at room temperature and are temperature-dependent, ranging from 1.86 to 4.07 B.M. at 80ºk, depending on the nature of R¹ and R² This behaviour is postulated to be due to a thermal equilibrium between the nearly equi-energetic spin-paired (t5/2g) and spin-free (t3/2ge2/g) configurations of the iron atom, resulting from the approximately equal magnitudes of the ligand field (Δ) and the pairing energy (π) in these complexes. The ligand field and consequently the magnetic behaviour are sensitive to the nature of R₁ and R₂: electron-withdrawing groups appear to be the most effective in increasing the population of the spin-paired configuration.