Static and dynamic hyperfine coupling of FE(II) in the FeS₄ unit. Results of Mössbauer spectroscopy on a [P(C₆H ₅)₄]₂[Fe(SC₆H₅) ₄] monocrystal

The hyperfine coupling parameters of Fe(II) in the FeS₄ unit of the complex anion [Fe(SC₆H₅)₄]2^- have been determined from an analysis of Mössbauer spectra of a monocrystal in the temperature range of 1.5 to 20 K and in magnetic fields of O to 6.7 T. Dynamic spin fluctuation effects have been included in the analysis by a stochastic model with spin-lattice relaxation. Anisotropy in the relaxation matrix element was of moderate significance and apparent only at low fields. The use of a monocrystal allows more precise determination of hyperfine parameters, because it avoids the averaging procedures which are inherent in studies on polycrystalline material. The set of parameters is essentially identical with that reported for reduced rubredoxin and indicates that the electronic ground state of [Fe(SC₆H₅)₄]^2-, predominantly of [3z² - r²] character, simulates very well the ground state of the protein FeS core.