An intermediate (C) that is observed in both phenol hydroxylation and catechol oxidation with the side-on peroxide species [Cu2O2(DBED)(2)](2+) (DBED = N-1,N-2-di-tert-butylethane-1,2-diamine) is identified as a copper(II) semiquinone species ([1]) through independent synthesis and characterization. The reaction of the redox-active 3,5-di-tert-butylquinone ligand with [(DBED)Cu-I(MeCN)](+) yields a copper(II) semiquinone [1](+) complex with a singlet ground state and an intense purple chromophore (epsilon(580) similar to 3500 M-1 cm(-1)). All other copper(II) semiquinone complexes characterized to date are paramagnetic and weakly colored (epsilon(800) similar to 500 M-1 cm(-1)). Antiferromagnetic coupling between the Cu-II center and the semiquinone radical in [1](+) is characterized by paramagnetic H-1 NMR and SQUID magnetometry. Comparative X-ray crystal structures along with density functional theory calculations correlate the geometric structures of copper(II) semiquinone complexes with their magnetic and optical properties. The unique observable properties of [1] originate from an increase in the overlap of the Cu 3d and semiquinone pi orbitals resulting from a large rhombic distortion in the structure with a twist of 51 degrees, attributable to the large isotropic demands of the tert-butyl substituents of the DBED ligand. Independent characterization of [1](+) allows the spectroscopic yields of intermediate C to be quantified in this intriguing hydroxylation reaction.