X-ray absorption spectroscopy at the sulfur K-edge (∼2470 eV) has been applied to a series of
2Fe−2S model complexes to obtain insight into their electronic structures. Since these 2Fe−2S complexes
contain both terminal thiolates and bridging sulfides, contributions to covalency from both sets of ligands can
be evaluated. Importantly, the pre-edge feature of sulfide can be resolved from that of thiolate due to differences
in effective nuclear charge. In our previous studies, the covalency of the metal−thiolate bond in [Fe(SR)4]-
was determined. In this study, sulfide covalency is quantified for the first time on the basis of an analysis of
previous X-ray photoelectron and X-ray absorption spectroscopic studies of [FeCl4]- which are then applied
to the bis-μ2-sulfide compound KFeS2. With references for both sulfide and thiolate covalencies thus established
for open d-shell systems, comparisons are made between thiolate and sulfide bonding. Sulfide−Fe covalency
in the [Fe2S2(SR)4]2- complexes is higher than thiolate−Fe covalency, indicating extensive charge donation
of the bridging sulfides. Finally, this investigation of model complexes is extended to the oxidized and reduced
2Fe−2S cluster of the Rieske protein of Paracoccus denitrificans which has terminal thiolates on one Fe
center, and histidines on the other Fe center. It is determined that thiolate covalency of the Fe(III) center is the
same in both the oxidized and reduced Rieske clusters and similar to that of the [Fe2S2(SR)4]2- model complexes.
Further, in the fully oxidized Rieske cluster, the sulfide covalency of the ferric center containing terminal
histidine ligation is ∼18% higher than the Fe(III) containing terminal thiolate ligation. This is consistent with
the fact that the histidine ligands are poorer donors and supports the suggestion that the terminal histidine
ligation makes a significant contribution to the higher reduction potential of the Rieske protein.
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