For O-2-reactivity studies aimed at modeling aspects of heme-copper oxidase enzyme function, use of heterobinucleating ligands (6-pyridyl (L-6) or 5-pyridyl (L-5) connection) gives rise to iron(II) (with "empty-tether"), 2a and 2b, and [LFeII... Cu-I](+) complexes, 3a and 3b, respectively. Reduced complexes 2 and 3 can exist in high-, intermediate-, or low-spin iron(II) configurations, depending on the solvent employed. Reaction of dioxygen with both 3a and 3b leads to a biomimetic reductive O-O bond cleavage forming mu-oxo species [(L)Fe-III-O-Cu-II](+) (4a, 4b). The X-ray structure of 4a is reported.