Crystal Structure and Electron Configuration of {MNO}⁸ Heme Complexes

Although research on nitrosyl (NO) heme complexes and their one-electron reduced form, nitroxyl (or nitroxyl anion, NO^-) derivatives, has been going on for decades, there are still disagreements about the electrical configuration of nitroxyl complexes, and the majority of the work on this topic is based on theoretical calculations. Following the initial nitroxyl iron porphyrin crystal structure, we present two further polymorphic forms of [CoCp₂][Fe(TFPPBr₈)(NO)]. Using the same completely halogenated porphyrin ligand, we also present two polymorphic forms of nitrosyl cobalt(II) complexes, which are another sort of {MNO}⁸ structure. In addition to the EXANES and EPR studies of these {FeNO}⁷ and {CoNO}⁸ complexes, the {FeNO}⁸ [CoCp₂][Fe(TFPPBr₈)(NO)] complex is also investigated by temperature-dependent Mössbauer experiments for the first time with the {FeNO}⁷ precursor as a control sample. The analysis of the Mössbauer and crystal structural parameters between these two types of {MNO}⁸ (M = Fe or Co) species and previously reported analogous ones allow us to conclude that the electronic configuration of [Fe(TFPPBr₈)(NO)]^- is best described as an intermediate between low-spin Fe(II)-NO^- and Fe(I)-NO^•.