Abstract
In addition to probing the formation of dinitrosyl iron complexes (DNICs) by the characteristic Fe K-edge pre-edge absorption energy ranging from 7113.4 to 7113.8 eV, the distinct S K-edge pre-edge absorption energy and pattern can serve as an efficient tool to unambiguously characterize and discriminate mononuclear DNICs and dinuclear DNICs containing bridged-thiolate and bridged-sulfide ligands. The higher Fe-S bond covalency modulated by the stronger electron-donating thiolates promotes the Fe → NO π-electron back-donation to strengthen the Fe-NO bond and weaken the NO-release ability of the mononuclear DNICs, which is supported by the Raman ν(Fe-NO) stretching frequency. The Fe-S bond covalency of DNICs further rationalizes the binding preference of the {Fe(NO) <sub>2</sub> } motif toward thiolates following the trend of [SEt] <sup>-</sup> > [SPh] <sup>-</sup> > [SC <sub>7</sub> H <sub>4</sub> SN] <sup>-</sup> . The relative d-manifold energy derived from S K-edge XAS as well as the Fe K-edge pre-edge energy reveals that the electronic structure of the {Fe(NO) <sub>2</sub> } <sup>9</sup> core of the mononuclear DNICs [(NO) <sub>2</sub> Fe(SR) <sub>2</sub> ] <sup>-</sup> is best described as {Fe <sup>III</sup> (NO <sup>-</sup> ) <sub>2</sub> } <sup>9</sup> compared to [{Fe <sup>III</sup> (NO <sup>-</sup> ) <sub>2</sub> } <sup>9</sup> -{Fe <sup>III</sup> (NO <sup>-</sup> ) <sub>2</sub> } <sup>9</sup> ] for the dinuclear DNICs [Fe <sub>2</sub> (μ-SEt)(μ-S)(NO) <sub>4</sub> ] <sup>-</sup> and [Fe <sub>2</sub> (μ-S) <sub>2</sub> (NO) <sub>4</sub> ] <sup>2-</sup> . © 2011 American Chemical Society.