Inspired by the chemistry relevant to dioxygen storage, transport and activation by metalloproteins, in particular for heme/copper oxidases, and carbon monoxide binding to metal-containing active sites as a probe or surrogate for dioxygen binding, a series of heme derived dioxygen and CO complexes have been designed, synthesized, and characterized with respect to their physical properties and reactivity. The focus of this study is in the description and comparison of three types heme–superoxo and heme–CO adducts. The starting point is in the characterization of the reduced heme complexes, [(F8)FeII], [(PPy)FeII] and [(PIm)FeII], where F8, PPy and PIm are iron(II)–porphyrinates and where PPy and PIm possess a covalently tethered axial base pyridyl or imidazolyl group, respectively. The spin-state properties of these complexes vary with solvent. The low temperature reactions between O2 and these reduced porphyrin FeII complexes yield distinctive low spin heme–superoxo adducts. The dioxygen binding properties for all three complexes are shown to be reversible, via alternate argon or O2 bubbling. Carbon monoxide binds to the reduced heme–FeII precursors to form low spin heme–CO adducts. The implications for future investigations of these heme O2 and CO adducts are discussed.Graphical abstractInspired by the chemistry relevant to dioxygen processing by metalloproteins, in particular for heme/copper oxidases, a series of heme derived dioxygen and CO complexes have been designed, synthesized, and characterized with respect to their physical properties and reactivity. The focus of this study is the description and comparison of three types of heme–superoxo and heme–CO adducts. Where the porphyrinates are one without an axial base covalently attached, while the others possess a covalently tethered axial base pyridyl or imidazolyl group.Highlights► New pyridyl or imidazolyl tethered heme–FeII, heme–FeIII–superoxo and heme–FeII–CO complexes have been generated and characterized. ► The new heme–Fe–superoxo and heme–Fe–CO complexes are determined to be six coordinate low spin complexes by using 1H or 2H NMR spectroscopy. ► Dioxygen (O2) binding to reduced base tethered heme–FeII complexes is reversible.