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 2) 3 - n(4-NO 2PyCH 2) nN] (n ≥ 0 - 3), were examined by X-ray absorption fine-structure and variable-temperature 1H 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). © 2011 American Chemical Society.