Bis (pyridine) (9, 10-phenanthrenequinone) (9, 10-phenanthrenediolato)copper(II), Cu(py)(2)(PhenCat)(PhenBQ), has been prepared by treating copper metal with 9,10-phenanthrenequinone in pyridine solution. In dilute solution, both Cu(py)(2)(PhenCat)(PhenBQ) and the related complex Cu(tmeda)(PhenCat)(PhenBQ) lose PhenBQ to form (CuL2)-L-II(PhenCat), where L-2= tmeda, 2 py. EPR spectra recorded at temperatures between 300 and 77 K reveal the presence of species with radical and metal localized spins together at equilibrium. Equilibria between (CuL2)-L-II(PhenCat) and (CuL2)-L-I(PhenSQ) redox isomers are solvent dependent, with a shift to higher temperature for polar solvents. Both complexes are oxygen sensitive, reacting with dioxygen to give complexes of diphenic acid. Structural characterization on products obtained with tmeda show that dioxygen insertion across the C-C bond within the chelate ring leads to dimeric products with adjacent Cu(II) ions bridged by diphenate ligands. The addition Of O-2 to Cu(tmeda)(PhenCat) in acetonitrile solution at 0 degreesC appears to form a peroxo complex, tentatively identified as Cu(tmeda)(O-2)(PhenQ) on the basis of iodometric titration, as the precursor to the diphenate complex.