Stable Ferrous Mononitroxyl {FeNO} 8 Complex with a Hindered Hydrotris(pyrazolyl)borate Coligand: Structure, Spectroscopic Characterization, and Reactivity Toward NO and O 2 .

The iron(II)-nitroxyl complex [Fe(NO)(L3)] (1) (with L3 ^- = a hindered hydrotris(pyrazolyl)borate ligand), a high-spin (hs)-{FeNO} ⁸ complex in the Enemark-Feltham notation, is surprisingly stable and is the first of its kind that could be structurally characterized. We further studied this compound using a variety of spectroscopic methods. These results indicate a hs iron(II) center with a bound ³ NO ^- ligand where the spins are antiferromagnetic coupled ( S _t = 1). Vibrational data show that this complex has a very strong Fe-NO bond. DFT calculations support this result and link it to very strong π-donation from the ³ NO ^- ligand to the iron(II) center. Furthermore, a very unusual equilibrium between the hs-{FeNO} ⁸ complex and a dinitrosyl iron complex (DNIC) of {Fe(NO) ₂ } ⁹ type is observed. The O ₂ reactivity of the complex is finally reported.