Revisiting NHC–Metal Bonding: π-Donation in Mid- to High-Valent Iron Nitrido Complexes Stabilizes the Fe(VI) Oxidation State

N-anchored tripodal N-heterocyclic carbene ligands, sat TIMMN Mes and Me2 TIMMN Mes, were synthesized and used to stabilize a series of mid-valent Fe(IV) and high-valent Fe(V) and Fe(VI) nitrides. The Fe(IV) and Fe(V) species adopt trigonal-pyramidal and trigonal-bipyramidal geometries, respectively, whereas the Fe(VI) nitrides exhibit octahedral coordination, representing rare examples of structurally characterized iron(VI) nitrido complexes. All complexes were characterized by single-crystal X-ray diffraction, multinuclear ( 1 H, 13 C, 15 N, and 19 F) NMR, zero- and applied-field 57 Fe Mössbauer, electron paramagnetic resonance, as well as vibrational and electronic absorption spectroscopy. Combined spectroscopic, electrochemical, and computational studies examined how systematic variation in the NHC backbone modulates the electronic structures of the [Fe≡N] n + and Fe–C NHC moieties. Structural and spectroscopic parameters of Me2 TIMMN Mes -based Fe(IV–VI) nitrides closely resemble those of parent TIMMN Mes analogues, while more pronounced deviations are observed for sat TIMMN Mes derivatives. 57 Fe Mössbauer spectroscopy at 80 K revealed an unusually negative isomer shift for the tetravalent ( d 4, S = 0) [( sat TIMMN Mes )Fe IV ≡N] + (δ = −0.45 mm s –1 ), distinct from Me2 TIMMN Mes and TIMMN Mes analogues (δ = −0.36 and −0.35 mm s –1 ), attributable to enhanced Fe–C NHC covalency in sat TIMMN Mes ligand. Cyclic voltammetry, supported by theoretical calculations, quantifies relative ligand π-donation in the Fe(IV) nitrido complexes, complementary to established NHC σ-donor descriptors, yielding the trend sat TIMMN Mes < TIMMN Mes < Me2 TIMMN Mes . Consistent with this trend, structural and EPR data indicate differing degrees of Jahn–Teller distortion among the paramagnetic Fe(V) nitrides ( d 3, S = 1/2). In contrast, the structural and spectroscopic differences are relatively diminished in highly covalent Fe(VI) nitrides ( d 2, S = 0).