The bis(mu-oxo) dimeric complexes {(ArOM)-O-iPr8(mu-O)}(2) (AriPrs = C6H-2,6-(C6H2-2,4,6-Pr-i(3))2-3,5-Pr-i(2); M = Fe (1), Co (2)) were prepared by oxidation of the M(I) half-sandwich complexes {(ArM)-M-iPr8(eta(6)-arene)} (arene = benzene or toluene). Iron species 1 was prepared by reacting {(ArFe)-Fe-iPr8 (eta(6)-benzene)} with N2O or O-2, and cobalt species 2 was prepared by reacting {(ArCo)-Co-iPr8(eta(6)-toluene)} with O-2. Both 1 and 2 were characterized by X-ray crystallography, UV-vis spectroscopy, magnetic measurements, and, in the case of 1, Mossbauer spectroscopy. The solid-state structures of both compounds reveal unique M-2(mu-O)(2) (M = Fe (1), Co(2)) cores with formally three-coordinate metal ions. The Fe center dot center dot center dot Fe separation in 1 bears a resemblance to that in the Fe-2(mu-O)(2) diamond core proposed for the methane monooxygenase intermediate Q The structural differences between 1 and 2 are reflected in rather differing magnetic behavior. Compound 2 is thermally unstable, and its decomposition at room temperature resulted in the oxidation of the Ar-iPr8 ligand via oxygen insertion and addition to the central aryl ring of the terphenyl ligand to produce the 5,5'-peroxybis[4,6-Pr-i(2)-3,7-bis(2,4,6-Pr-i(3)-phenyl)oxepin-2(5H)-one] (3). The structure of the oxidized terphenyl species is closely related to that of a key intermediate proposed for the oxidation of benzene.