Investigation of the electronic structure of 2Fe-2S model complexes and the rieske protein using ligand K-edge X-ray absorption spectroscopy

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 thiolares 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.