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)]i occupancies (where i = 0–4 for mononitrosyl complexes) identified the dominant electronic configurations of {FeNO}6 and {FeNO}7 complexes as FeIII–NO0 and FeII–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 FeI–NO0 and FeII–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 FeI–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 wi 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.