An unusual aspect of nitrile hydratase (NHase), a non-heme, redox inactive enzyme stabilized in the +3 oxidation state, and ligated by two peptide amides and three cysteinates, is that two of the three coordinated cysteinate sulfurs appear to be post-translationally oxidized- one to a sulfenic acid (¹¹⁴Cys-S-(OH)) and the other to a sulfinate (¹¹²Cys-S=(O)₂). Insight regarding the functional role of these oxidized cysteinates would require isolation of the unmodified form of NHase; however, this form of the enzyme has yet to be characterized. Herein we report the synthesis of an unmodified NHase analogue, [Fe^III(tame–N₂S₃)]^2– (1). In contrast to modified NHase (which is S= 1/2, readily binds H₂O, and displays a S-to-Fe CT band at 710 nm), 1 is intermediate-spin S=3/2, does not readily bind axial s-donor ligands, and has a substantially blue-shifted S-to-Fe CT band (l_max= 475 nm). Electrophilic and p–acceptor ligands O₂, NO, and HBF₄ do react with 1, on the other hand. The strong equatorial ligand field, coupled with the electron–rich environment, afforded in the absence of oxygenated thiolates, decreases the Lewis acidic properties of the vacant apical site, to the point where “substrates” do not bind. Post–translational modification thus appears to regulate NHase activity by turning on substrate binding.