Treatment of [Fe-IV(O)(TPA)(NCMe)](CF3SO3)(2) [TPA, N, N, N-tris(2-pyridylmethyl) amine] with 3 equiv of NR4X (X = CF3CO2, Cl, or Br) in MeCN at -40 degrees C affords a series of metastable [FeIV(O)(TPA)(X)](+) complexes. Some characteristic features of the S = 1 oxoiron(IV) unit are quite insensitive to the ligand substitution in the equatorial plane, namely, the Fe-O distances (1.65-1.66 angstrom), the energy (similar to 7114.5 eV) and intensity [25(2) units] of the 1s-to-3d transition in the X-ray absorption spectra, and the Mossbauer isomer shifts (0.01-0.06 mm center dot s(-1)) and quadrupole splittings (0.92-0.95 mm, s(-1)). The coordination of the anionic X ligand, however, is evidenced by red shifts of the characteristic near-IR ligand-field bands (720-800 nm) and spectroscopic observation of the bound anion by F-19 NMR for X = CF3CO2 and by EXAFS analysis for X = Cl (r(Fe-Cl) = 2.29 angstrom) and Br (r(Fe-Br) = 2.43 angstrom). Density functional theory calculations yield Mossbauer parameters and bond lengths in good agreement with the experimental data and produce excited-state energies that follow the trend observed in the ligand-field bands. Despite mitigating the high effective charge of the iron(IV) center, the substitution of the MeCN ligand with monoanionic ligands X- decreases the thermal stability of [FeIV(O)(TPA)](2+) complexes. These anion-substituted complexes model the cis-X-Fe-IV = O units proposed in the mechanisms of oxygen-activating nonheme iron enzymes.