A mononuclear oxoiron(iv) complex 1-trans bearing two equatorial sulfur ligations is synthesized and characterized as an active-site model of the elusive sulfur-ligated FeIV[double bond, length as m-dash]O intermediates in non-heme iron oxygenases. The introduction of sulfur ligands weakens the Fe[double bond, length as m-dash]O bond and enhances the oxidative reactivity of the FeIV[double bond, length as m-dash]O unit with a diminished deuterium kinetic isotope effect, thereby providing a compelling rationale for nature's use of the cis-thiolate ligated oxoiron(iv) motif in key metabolic transformations.