π Back-Bonding of Iron(II) Complexes Supported by Tris(pyrid-2-ylmethyl)amine and Its Nitro-Substituted Derivatives

The electronic and geometric structures of a series of iron(II) complexes supported by tetradentate tris(pyrid-2-ylmethyl)amine-type ligands with different numbers of 4-nitropyridine groups, [(PyCH<sub>2</sub>)<sub>3–<i>n</i></sub>(4-NO<sub>2</sub>PyCH<sub>2</sub>)<sub><i>n</i></sub>N] (<i>n</i> = 0–3), were examined by X-ray absorption fine-structure and variable-temperature <sup>1</sup>H NMR spectroscopies and theoretical calculations to reveal how the low-spin state is stabilized through π back-bonding interactions between iron(II) and 4-nitropyridine donor group(s).