The Unusual Electronic Structure of Dinitrosyl Iron Complexes

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 (S(Fe) = 5/2) antiferromagnetically coupled to two NO(-) ligands (S((NO)(2)) = 2) and a HS ferrous ion (S(Fe) = 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 (S(Fe) = 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 pi-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.