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Abstract:
Dinitrosyl iron complexes (DNICs) are implicated in the degradation and reassembly chemistry of iron−sulfur clusters; however, their electronic structure is not well understood. Here, experimentally validated electronic structures of a {Fe(NO)2}9 species and its one-electron reduced form, {Fe(NO)2}10, were reached through a detailed analysis of the Kohn−Sham density functional solutions that successfully reproduce the experimental structures and spectroscopic parameters. The {Fe(NO)2}9 unit is best rationalized by a resonance hybrid consisting of a HS ferric center (SFe = 5/2) antiferromagnetically coupled to two NO− ligands (S(NO)2 = 2) and a HS ferrous ion (SFe = 2) coupled to an overall 4(NO)2− ligand (S(NO)2 = 3/2) in an antiferromagnetic fashion. The {Fe(NO)2}10 species is best interpreted as a HS ferrous center (SFe = 2) that is antiferromagnetically coupled to two triplet NO− ligands (S(NO)2 = 2). A salient feature of this electronic structure description is the very covalent bonding involving the iron center and the two NO ligands. As a result, a “one-above-four” ligand field splitting pattern is identified in DNICs, in which four of the five Fe-3d orbitals are strongly π-bonding MOs with respect to the Fe−NO interaction while the last Fe 3d-based orbital remains essentially nonbonding. The latter acts as the electron acceptor orbital for the one-electron reduction of the {Fe(NO)2}9 species. This unusual ligand field splitting pattern may have mechanistic implications for the degradation and reassembly chemistry of iron−sulfur clusters involving DNICs.
