Low-temperature magnetochemical and spectroscopic studies of variable-spin tris(monothiocarbamato)iron(III) and tris(diselenocarbamato)iron(III) complexes

Abstract

Two tris(dialkylmonothiocarbamato)iron(III) complexes, [Fe(OSCNR₂)₃], with FeS₃O₃ cores and R = Me and Et, have been investigated in the solid state by low-temperature (4.2–80 K) magnetic susceptibility and ⁵⁷Fe Mössbauer spectroscopy measurements and in solid solution by e.s.r. spectroscopy. The magnetic and Mössbauer studies confirm our earlier report that the complexes [Fe(OSCNMe₂)₃] and [Fe(OSCNEt₂)₃] are new ²T(S=½, low-spin)⇌⁶A(S= 5/2, high-spin) spin-equilibrium species, although [Fe(OSCNMe₂)₃] can also be obtained in a crystalline modification that remains totally high-spin down to ca. 10 K. The complex [Fe(OSCNMe₂)₃], with the ²T and ⁶A states nearly equally populated, gives a Mössbauer spectrum at 4.2 K that appears as an ‘average’ of the two spin states, suggesting that the rates of ²T⇌⁶A spin interconversion are ≳ 10⁷ s^–1. The e.s.r. spectra at 12 K of the two compounds in solvent glasses display only one signal at g ca. 4.3 which probably arises from rhombically distorted high-spin iron(III). Finally, solid-solution state e.s.r. spectra for four known ²T⇌⁶A tris (disubstituted diselenocarbamato)iron(III) complexes, [Fe(Se₂CNR₂)₃], with FeSe₆ cores are also reported; the spectra are complex with g values of approximately 2.0, 3.0, and 4.3. The four FeSe₆ centres are essentially Mössbauer silent, probably due to a selenium edge absorption at 12.6 keV or to effective γ-ray scattering by the selenium donor atoms.