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Abstract

The reaction of low-spin [Fe^IICl(cyclam-ac)] with NO in CH₃CN yields the octahedral, non-heme (nitrosyl)iron complex [Fe(NO)(cyclam-ac)](PF₆) (2), S = 1/2, which can be one-electron oxidized with (tris(4-bromophenyl)ammoniumyl)hexachloroantimonate to the complex [Fe(NO)(cyclam-ac)](PF₆)₂ (1‘), S = 0, or complex [Fe(NO)(cyclam-ac)]Cl(ClO₄)·H₂O (1); (cyclam-ac)^- represents the pentadentate monoanion of 1,4,8,11-tetraazacyclotetradecane-1-acetic acid. Similarly, in CH₃CN solution 2 can be electrochemically or chemically reduced by one-electron to the neutral complex [Fe(NO)(cyclam-ac)]⁰ (3), S = 0. 1−3 represent members of the {FeNO}^6-8 series, respectively. The electronic structure of these three species have been spectroscopically elucidated by EPR, UV−vis, Mössbauer, and IR spectroscopy. The crystal structures of 1 and 2 have been determined by X-ray crystallography. In addition, detailed density functional calculations have been performed for all three species taking into account the possibility that 3 is actually a protonated HNO or NOH species. For all complexes structures, energetics, IR parameters, and Mössbauer parameters as well as EPR parameters (g-values and hyperfine couplings) have been calculated using state-of-the art DFT methods which are compared to experiment. The results establish unequivocally that 3 is indeed the elusive {FeNO}⁸ species. Furthermore a detailed picture of the bonding in the low-spin non-heme iron nitrosyl series {FeNO}⁶, {FeNO}⁷, and {FeNO}⁸ has been developed. This allows a description of 1 as a low-spin ferrous complex containing an N-coordinated NO⁺, whereas 2 is a low-spin ferrous species with a NO^• ligand and 3 is a low-spin ferrous complex with a coordinated NO^-. On the basis of this description, all spectroscopic and geometric observables find a satisfactory interpretation.