Cyclopentadienyl and pentamethylcyclopentadienyl ruthenium(II) complexes have been synthesized with cyclic (RPCH2NR'CH2)(2) ligands, with the goal of using these [Cp-R '' Ru-(p(2)(R)N(2)(R)')](+) complexes for catalytic O-2 reduction to H2O (R = t-butyl, phenyl; R' = benzyl, phenyl; R '' = methyl, H). In each compound, the Ru is coordinated to the two phosphines, positioning the amines of the ligand in the second coordination sphere where they may act as proton relays to a bound dioxygen ligand. The phosphine, amine, and cyclopentadienyl substituents have been systematically varied in order to understand the effects of each of these parameters on the properties of the complexes. These Cp-R '' Ru((P2NR2R)-N-R')(+) complexes react with O-2 to form eta(2)-peroxo complexes, which have been characterized by NMR, IR, and X-ray crystallography. The peak reduction potentials of the O-2 ligated complexes have been shown by cyclic voltammetry to vary as much as 0.1 V upon varying the phosphine and amine. In the presence of acid, protonation of these complexes occurs at the pendent amine, forming a hydrogen bond between the protonated amine and the bound O-2. The ruthenium peroxo complexes decompose upon reduction, precluding catalytic O-2 reduction. The irreversible reduction potentials of the protonated O-2 complexes depend on the basicity of the pendent amine, giving insight into the role of the proton relay in facilitating reduction.