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

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)] occupancies (where i = 0-4 for mononitrosyl complexes) identified the dominant electronic configurations of {FeNO}⁶ and {FeNO}⁷ complexes as Fe^III-NO⁰ and Fe^II-NO⁰, 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-NO⁰ 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 Fe⁰-NO^- configuration. Based on the weights w of the [π*(NO)] 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.