Modified His-His dipeptides have been reacted with copper(I) salts to model active-site Cu ions bound by contiguous His residues in certain oxygen-activating copper proteins, as well as amyloid beta-peptide. Chelation of copper(I) by these ligands affords linear, two-coordinate complexes as studied structurally by X-ray absorption spectroscopy. The complexes are robust toward oxidation, showing limited to no reactivity with O-2, and they bind CO weakly. Reaction with a third ligand (N-methylimidazole) affords complexes with a markedly different structure (distorted T-shaped) and reactivity, binding CO and oxidizing rapidly upon exposure to dioxygen.