NO Oxidation States in Nonheme Iron Nitrosyls: A DMRG-CASSCF Study of {FeNO}⁶–¹⁰ Complexes

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High Resolution Image Download MS PowerPoint Slide Building upon an earlier study of heme-nitrosyl complexes ( Inorg. Chem . 2023, 62, 20496–20505), we examined a wide range of nonheme {FeNO} 6–10 complexes (the superscript represents the Enemark-Feltham count) and two dinitrosyl iron complexes using DMRG-CASSCF calculations. Analysis of the wave functions in terms of resonance forms with different [π*(NO)] i occupancies (where i = 0–4 for mononitrosyl complexes) identified the dominant electronic configurations of {FeNO} 6 and {FeNO} 7 complexes as Fe III –NO 0 and Fe II –NO 0, respectively, mirroring our previous findings on heme-nitrosyl complexes. A trigonal-bipyramidal S = 1 {FeNO} 8 complex with an equatorial triscarbene ligand set appears best described as a resonance hybrid of Fe I –NO 0 and Fe II –NO – . Reduction to the corresponding S = 1/2 {FeNO} 9 state was found to involve both the metal and the NO, leading to an essentially Fe I –NO – complex. Further reduction to the {FeNO} 10 state was found to be primarily metal-centered, leading to a predominantly Fe 0 –NO – configuration. Based on the weights w i of the [π*(NO)] i resonance forms, an overall DMRG-CASSCF-based π*(NO) occupation number could be derived, which was found to exhibit a linear correlation with both the NO bond distance and NO stretching frequency, allowing a readout of the NO oxidation state from the NO bond distance.