| Sumario: | Iron L-edge, iron L-edge, and sulfur K-edge X-ray absorption spectroscopy was performed on a series of compounds [FeHbuea(X)] (X = S, O, OH). The experimentally determined electronic structures were used to correlate to density functional theory calculations. Calculations supported by the data were then used to compare the metalligand bonding and to evaluate the effects of H-bonding in FeO vs FeS complexes. It was found that the FeO bond, while less covalent, is stronger than the FeS bond. This dominantly reflects the larger ionic contribution to the FeO bond. The H-bonding energy (for three H-bonds) was estimated to be -25 kcal/mol for the oxo as compared to -12 kcal/mol for the sulfide ligand. This difference is attributed to the larger charge density on the oxo ligand resulting from the lower covalency of the FeO bond. These results were extended to consider an FeO complex with the same ligand environment. It was found that hydrogen bonding to FeO is less energetically favorable than that to FeO, which reflects the highly covalent nature of the FeO bond.
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