The syntheses and structures of three new coordinatively unsaturated, monomeric, square-pyramidal thiolateligated Fe(III) complexes are described, [Fe-III((tame-N-3)S-2(Me2))](+) (1), [Fe-III(Et-N2S2Me2)(py)](1-) (3), and [Fe-III((tame-N2S)S-2(Me2))](2-) (15). The anionic bis-carboxamide, tris-thiolate N2S3 coordination sphere of 15 is potentially similar to that of the yet-to-be characterized unmodified form of NHase. Comparison of the magnetic and reactivity properties of these reveals how anionic charge build up (from cationic 1 to anionic 3 and dianionic 15) and spin-state influence apical ligand affinity. For all of the ligand-field combinations examined, an intermediate S = 3/2 spin state was shown to be favored by a strong N2S2 basal plane ligand field, and this was found to reduce the affinity for apical ligands, even when they are built in. This is in contrast to the post-translationally modified NHase active site, which is low spin and displays a higher affinity for apical ligands. Cationic 1 and its reduced Fell precursor are shown to bind NO and CO, respectively, to afford [Fe-III((tame-N-3)S-2(Me))(NO)](+) (18, nu(NO) = 1865 cm(-1)), an analogue of NO-inactivated NHase, and [Fe-II((tame-N-3)S-2(Me))(CO)] (16; nu(CO) stretch (1895 cm(-1)). Anions (N-3(-), CN-) are shown to be unreactive toward 1, 3, and 15 and neutral ligands unreactive toward 3 and 15, even when present in 100-fold excess and at low temperatures. The curtailed reactivity of 15, an analogue of the unmodified form of NHase, and its apical-oxygenated S = 3/2 derivative [Fe-III((tame-N2SO2)S-2(Me2))](2-) (20) suggests that regioselective post-translational oxygenation of the basal plane NHase cysteinate sulfurs plays an important role in promoting substrate binding. This is supported by previously reported theoretical (DFT) calculations.