Crystal Structure and Electron Configuration of {MNO}<sup>8</sup> Heme Complexes

Although\nresearch on nitrosyl (NO) heme complexes and their one-electron\nreduced form, nitroxyl (or nitroxyl anion, NO<sup>–</sup>)\nderivatives, has been going on for decades, there are still disagreements\nabout the electrical configuration of nitroxyl complexes, and the\nmajority of the work on this topic is based on theoretical calculations.\nFollowing the initial nitroxyl iron porphyrin crystal structure, we\npresent two further polymorphic forms of [CoCp<sub>2</sub>][Fe(TFPPBr<sub>8</sub>)(NO)]. Using the same completely halogenated porphyrin ligand,\nwe also present two polymorphic forms of nitrosyl cobalt(II) complexes,\nwhich are another sort of {MNO}<sup>8</sup> structure. In addition\nto the EXANES and EPR studies of these {FeNO}<sup>7</sup> and {CoNO}<sup>8</sup> complexes, the {FeNO}<sup>8</sup> [CoCp<sub>2</sub>][Fe(TFPPBr<sub>8</sub>)(NO)] complex is also investigated by temperature-dependent\nMössbauer experiments for the first time with the {FeNO}<sup>7</sup> precursor as a control sample. The analysis of the Mössbauer\nand crystal structural parameters between these two types of {MNO}<sup>8</sup> (M = Fe or Co) species and previously reported analogous\nones allow us to conclude that the electronic configuration of [Fe(TFPPBr<sub>8</sub>)(NO)]<sup>−</sup> is best described as an intermediate\nbetween low-spin Fe(II)–NO<sup>–</sup> and Fe(I)–NO<sup>•</sup>.