The bischelate monoanion [Fe((SCH2)2C6H4)2]1- contains an unconstrained, near-tetrahedral Fe(III)-S4 coordination unit (similar to the Fe-S4 coordinationunit occurring in rubredoxin proteins) and exhibits the Fe(III)/Fe(II) redox couple. Mossbauer parameters at 77 K are isomer shift δ = 0.13 mm/s (relative to iron metal) and electric quadrupole splitting ΔEQ = 0.57 mm/s. Application of an external magnetic field parallel to the direction of observation at 4.2 K induces a saturation magnetic hyperfine field of - 380 kOe. Upon reduction to the dianion [Fe((SCH2)2C6H4)2]2- the isomer shift changes to δ = 0.61 mm/s and the quadrupole splitting at 4.2 K to ΔEQ = 3.34 mmjs. From the temperature dependence of ΔEQ for 1.4 K < T < 295 K we infer the splitting of the ground orbital doublet, e(dz2, dx2-y2), to be Δ ≈ 900 cm-1. External magnetic fields up to 80 kOe reveal the sign of Vzz, to be negative indicating that the ground orbital has dz2 symmetry and induce a magnetic hyperfine interaction of negative sign. For H0 = 60 kOe, Hhf = - 128 kOe. | Hhf | increases with increasing H0, showing that saturation has not been reached up to H0 = 80 kOe, and implying a zero field splitting of several cm-1 of the electronic ground state. These results as well as magnetic moment measurements for the unconstrained model compounds are comparable to measurements in oxidized and reduced rubredoxin and imply that an «entatic state» description of rubredoxin is not valid.



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