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 have examined a wide range of nonheme {FeNO}6–10 complexes, as well as two dinitrosyl iron complexes (DNICs), using DMRG-CASSCF theory and DFT. We found striking parallels between heme and nonheme complexes in terms of molecular spin density profiles and the local oxidation state of the NO moiety. For spin densities, classic pure functionals were found to give the closest agreement to DMRG-CASSCF theory. Analysis of DMRG-CASSCF wave functions identified NO0 resonance forms as the largest contributors to both {FeNO}6 and {FeNO}7 complexes. Our results thus suggest that FeIII-NO0 and FeII-NO0 are the dominant electronic configurations for {FeNO}6 and {FeNO}7 complexes, respectively, mirroring our previous findings on heme-nitrosyl complexes. However, multiple strong-field ligands such as cyanide or thiolate bring about a substantial contribution from an NO− resonance form to the overall {FeNO}7 wave functions. A trigonal-bipyramidal S = 1 {FeNO}8 complex with an equatorial triscarbene ligand set was best described as a resonance hybrid of FeI-NO0 and FeII-NO−, echoing a similar description for the S = 0 {FeNO}8 heme complex {Fe[P](NO)}– (P = porphyrinato). Reduction to the corresponding S = 1/2 {FeNO}9 state was found to involve the whole molecule, i.e., 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. A linear correlation between the NO bond distance and NO stretching frequency was also observed, which could be mapped onto a similar correlation between the NO bond distance and NO π* occupancy, allowing a simple readout of the NO oxidation state from the NO bond distance.