Insight into chalcogenolate-bound {Fe(NO)2}9 dinitrosyl iron complexes (DNICs): covalent character versus ionic character

The synthesis, characterization and transformation of the thermally unstable {Fe(NO)(2)}(9) dinitrosyl iron complex (DNIC) [(OMe)(2)Fe(NO)(2)](-) (2) were investigated. The {Fe(NO)(2)}(9) DNIC 2 characterized by single-crystal X-ray diffraction is exclusively stabilized by the weak intermolecular [Fe(OMe)(2)(K+)] interactions (O(3)K(1) and O(4)K(1) distances of 2.818(3) and 2.810(3) angstrom, respectively). The binding affinity of chalcogenolate-containing ligands toward the {Fe(NO)(2)}(9) motif follows the series [SEt](-) > [SPh](-) > [OPh](-) > [OMe](-), which is dictated by the synergistic cooperation of the electron-donating order ([SEt](-) > [SPh](-) > [OPh](-)) and the soft-hard order (from soft to hard, [SEt](-) approximate to [SPh](-) > [OPh](-) > [OMe](-)). In comparison with the XAS Fe K-edge pre-edge energy of {Fe(NO)(2)}(9) [(RS)(2)Fe(NO)(2)](-) (R = Ph (4), Et (5)) and [(PhO)(2)Fe(NO)(2)](-) (6) DNICs falling within the reported range of 7113.4-7113.9 eV, the distinctive pre-edge energy of 7114.2 eV exhibited by complex 2 suggests that the electronic structure of {Fe(NO)(2)}(9) DNIC 2 may be qualitatively described as a {Fe-III(NO-)(2)}(9) electronic structure induced by the dominant ionic character of Fe-OMe bonds, instead of the resonance hybrids of {Fe-II(NO-)(NO)}(9) and {Fe-III(NO-)(2)}(9) electronic structures induced by the dominant metal-ligand covalency of {Fe(NO)(2)}(9) DNICs 4-6. As shown in TD-DFT computation, the increased population of NO ligands in MO 125 (45.1% NO) attenuating the OMe-induced polarization imposed on the Fe center through the delocalized covalent nature of Fe-NO bonds supports the lower/synergistic NO/OMe Fe charge transfer energy (1216 nm) observed in the solid-state UV-vis spectrum of complex 2 compared to those (1140 nm) of complexes 4-6.