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 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_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.

Analysis of the DMRG-CASSCF wave functions for a wide range of iron nitrosyls has allowed the determination of long-sought local oxidation states of the Fe center and the NO moiety. The picture that emerges appears to hold for heme and nonheme systems alike and regardless of spin state for a given Enemark-Feltham count.