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(TFPPBr8)(NO)], and the structure of the {FeNO}(7) precursor [Fe(TFPPBr8)(NO)] are determined at 100 K. The two complexes are also characterized by FTIR and UV/Vis spectroscopy. [Fe(TFPPBr8)(NO)](-) shows distinct structural features in contrast to a nitrosyl iron(II) porphyrinate on the Fe-N-O(-) moiety, which include a much more bent Fe-N-O(-) angle (122.4(3)°), considerably longer Fe-NO(-) (1.814(4)) and N-O(-) (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(TFPPBr8)(NO)](-) and [Fe(TFPPBr8)(NO)] suggest a low-spin state of the iron(II) atom at 100 K.