Selenium- and sulfur-containing compounds can act as antioxidants by binding copper. To determine how this copper coordination results in the observed antioxidant activity, biologically relevant Cu⁺ and Cu²⁺ complexes with the formulae [Cu(dmit)₃]⁺ (3), [Cu(dmise)₄]⁺ (4a), and [Tpm^iPrCu(MISeox)]²⁺ (6) (dmise = N,N′-dimethylimidazole selone; dmit = N,N′-dimethylimidazole thione; MISeox = bis(1-methylimidazolyl)diselenide; Tpm^iPr = tris(1,3-diisopropylpyrazolyl)methane) were synthesized, characterized, and their structures determined by single-crystal X-ray crystallography. In addition, kinetic studies using UV-vis spectroscopy indicate that dmise reduces Cu²⁺ to Cu⁺ three times faster than dmit. Coordination of dmise and MISeox to copper also results in more negative Cu^2+/+ reduction potentials (−373 mV and −503 mV) compared to dmit (−217 mV). These results highlight the different complexation behaviors and reactivities of analogous selone- and thione-containing compounds, traits which likely influence their antioxidant activity.