One Electron Makes Differences: From Heme {FeNO} 7 to {FeNO} 8

The first X‐ray single‐crystal structure of a {FeNO} 8 porphyrin complex [Co(Cp) 2 ][Fe(TFPPBr 8 )(NO)], and the structure of the {FeNO} 7 precursor [Fe(TFPPBr 8 )(NO)] are determined at 100 K. The two complexes are also characterized by FTIR and UV/Vis spectroscopy. [Fe(TFPPBr 8 )(NO)] − shows distinct structural features in contrast to a nitrosyl iron(II) porphyrinate on the FeNO − moiety, which include a much more bent FeNO − angle (122.4(3)°), considerably longer FeNO − (1.814(4)) and NO − (1.194(5) Å) bond distances. These and the about 180 cm −1 downshift ν N‐O stretch (1540 cm −1 ) can be understood by the covalently bonding nature between the iron(II) and the NO − ligand which possesses a two‐electron‐occupied π* orbital as a result of the reduction. The overall structural features of [Fe(TFPPBr 8 )(NO)] − and [Fe(TFPPBr 8 )(NO)] suggest a low‐spin state of the iron(II) atom at 100 K.