Highest Recorded N–O Stretching Frequency for 6‑Coordinate {Fe-NO}⁷ Complexes: An Iron Nitrosyl Model for His₃ Active Sites

We report the synthesis, structure, and reactivity of [Fe­(T1Et4iPrIP)­(OTf)₂] [<b>1</b>; T1Et4iPrIP = tris­(1-ethyl-4-isopropylimidazolyl)­phosphine]. Compound <b>1</b> reacts reversibly with nitric oxide to afford [Fe­(T1Et4iPrIP)­(NO)­(THF)­(OTf)]­(OTf) (<b>2</b>), which is the first example of a 6-coordinate {FeNO}⁷ <i>S</i> = ³/₂ complex containing a linear Fe–N–O group. <b>2</b> exhibits the highest ν­(NO) for compounds in this class. Density functional theory studies reveal an enhanced degree of β-electron transfer from π*­(NO) to the Fe d orbitals accounting for the large stretching frequency