The preference for the formation of a particular Cu2O2 isomer coming from (ligand)-Cu-I/O-2 reactivity can be regulated with the steric demands of a TMPA (tris(2-pyridylmethyl)amine) derived ligand possessing 6-pyridyl substituents on one of the three donor groups of the tripodal tetradentate ligand. When this substituent is an -XHR group (X = N or C) the traditional Cu-I/O-2 adduct forms a (mu-1,2)peroxodicopper(II) species (A). However, when the substituent is the slightly bulkier XR2 Moiety {aryl or NR2 (R not equal H)), a bis(mu-oxo)dicopper(III) structure (C) is favored. The reactivity of one of the bis(mu-oxo)dicopper(III) species, [{(6tbp)Cu-III)(2)(O2-)(2)](2+) (7-O-2) (6tbp = (6-'Bu-phenyl-2-pyridylmethyl)bis(2-pyridylmethyl)amine), was probed, and for the first time, exogenous toluene or ethylbenzene hydrocarbon oxygenation reactions were observed, Typical monooxygenase chemistry occurred: the benzaldehyde product includes an 18-O atom for toluene/7-O-18(2) reactivity, and a H-atom abstraction by 7-O-2 is apparent from study of its reactions with ArOH substrates, as well as the determination of k(H)/k(D) approximate to 7 in the toluene oxygenation (i.e., PhCH3 VS PhCD3 substrates). Proposed courses of reaction are presented, including the possible involvement of PhCH2OO center dot and its subsequent reaction with copper(I) complex, the latter derived from dynamic solution behavior of 7-O-2. External TMPA ligand exchange for copper in 7-O-2 and O-O bond (re)formation chemistry, along with the ability to protonate 7-O-2 and release of H2O2 indicate the presence of an equilibrium between [{(6tbp)Cu-III}(2)(O2-)(2)](2+) (7-O-2) and a (mu-1,2)peroxodicopper(II) form.