Spectroscopic studies combined with calculations are used to describe the electronic structure and vibrational properties of mononuclear four-coordinate end-on alkylperoxo and hydroperoxo Cu(II) complexes. EPR defines a Cu x(2)-y(2) ground state with similar to 62% Cu character. From absorption, MCD, and resonance Raman spectroscopies, the main bonding interaction between the alkyl(hydro)peroxide and Cu(LI) is found to involve the pi -donation of the alkyl(hydro)peroxide pi*(v) into the Cu x(2)-y(2) orbital, which dominates the observed spectroscopic features, producing an intense absorption band at similar to 16 600 cm(-1) (similar to 600 nm). On the basis of the vibrational frequencies, isotope shifts, and normal coordinate analyses, the dominant vibrations of the alkyl(hydro)peroxo complexes are assigned and the Cu-O and O-O force constants are determined. The observed strong Cu-O bond and the large alkyl(hydro)peroxide-to-Cu(II) charge donation are ascribed to the low coordination number of Cu and the distorted T-d ligand field. The observed strong O-O bond mainly derives from polarization by the alkylcarbon/proton. The unoccupied peroxide sigma* orbital is also greatly stabilized in energy, and the complexes are activated for electrophilic attack. Experimentally calibrated density functional calculations, coupled with frontier molecular orbital theory, are employed to obtain insight into the reactivity of these model complexes. Mechanisms of electrophilic attack, O-O bond cleavage, and H atom abstraction are evaluated, and their relevance to dopamine beta -monooxygenase and peptidylglycine alpha -hydroxylating monooxygenase reactivities is considered.