The bis(-oxo) dimeric complexes {AriPr8OM(-O)}2 (AriPr8 = C6H-2,6-(C6H2-2,4,6-iPr3)2-3,5-iPr2; M = Fe (1), Co (2)) were prepared by oxidation of the M(I) half-sandwich complexes {AriPr8M(η6-arene)} (arene = benzene or toluene). Iron species 1 was prepared by reacting {AriPr8Fe(η6-benzene)} with N2O or O2, and cobalt species 2 was prepared by reacting {AriPr8Co(η6-toluene)} with O2. Both 1 and 2 were characterized by X-ray crystallography, UV-vis spectroscopy, magnetic measurements, and, in the case of 1, Mössbauer spectroscopy. The solid-state structures of both compounds reveal unique M2(-O)2 (M = Fe (1), Co(2)) cores with formally three-coordinate metal ions. The Fe… Fe separation in 1 bears a resemblance to that in the Fe2(-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 AriPr8 ligand via oxygen insertion and addition to the central aryl ring of the terphenyl ligand to produce the 5,5'-peroxy-bis[4,6-iPr2-3,7-bis(2,4,6-iPr3-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.