Structural and Electronic Responses to the Three Redox Levels of Fe(NO)N 2 S 2 -Fe(NO) 2 .

The nitrosylated diiron complexes, Fe ₂ (NO) ₃ , of this study are interpreted as a mono-nitrosyl Fe(NO) unit, MNIU, within an N ₂ S ₂ ligand field that serves as a metallodithiolate ligand to a dinitrosyl iron unit, DNIU. The cationic Fe(NO)N ₂ S ₂ ⋅Fe(NO) ₂ ⁺ complex, 1 ⁺ , of Enemark-Feltham electronic notation {Fe(NO)} ⁷ -{Fe(NO) ₂ } ⁹ , is readily obtained via myriad synthetic routes, and shown to be spin coupled and diamagnetic. Its singly and doubly reduced forms, {Fe(NO)} ⁷ -{Fe(NO) ₂ } ¹⁰ , 1 ⁰ , and {Fe(NO)} ⁸ -{Fe(NO) ₂ } ¹⁰ , 1 ^- , were isolated and characterized. While structural parameters of the DNIU are largely unaffected by redox levels, the MNIU readily responds; the neutral, S= 1 / 2 , complex, 1 ⁰ , finds the extra electron density added into the DNIU affects the adjacent MNIU as seen by the decrease its Fe-N-O angle (from 171° to 149°). In contrast, addition of the second electron, now into the MNIU, returns the Fe-N-O angle to 171° in 1 ^- . Compensating shifts in Fe _MNIU distances from the N ₂ S ₂ plane (from 0.518 to 0.551 to 0.851 Å) contribute to the stability of the bimetallic complex. These features are addressed by computational studies which indicate that the MNIU in 1 ^- is a triplet-state {Fe(NO)} ⁸ with strong spin polarization in the more linear FeNO unit. Magnetic susceptibility and parallel mode EPR results are consistent with the triplet state assignment.
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