Copper-dioxygen (CUO2) adducts are frequently proposed as intermediates in enzymes, yet their electronic and vibrational structures have not always been understood. [Cu(eta(1)-O-2)TMG(3)tren](+) (TMG(3)-tren = 1,1,1-tris{2-[N-2-(1,1,3,3-tetramethylguanidino)]ethyl}amine) features end-on (eta(1)) O-2 coordination in the solid state. Described here is an investigation of the compound's solution properties by nuclear magnetic resonance spectroscopy, density functional calculations, and oxygen isotope effects. The study yields two major findings. First, [CU(eta(1)-O-2)TMG(3)tren](+) is paramagnetic due to a triplet electronic structure; this is in contrast to other copper compounds where O-2 is bound in a side-on manner. Second, the oxygen equilibrium isotope effect upon O-2 binding to copper(I) (O-18 EIE equivalent to K((OO)-O-16-O-16)IK((OO)-O-16-O-18) = 1.0148 +/- 0.0012) is significantly larger than those determined for iron and cobalt eta(1)-O-2 adducts. This result is suggested to reflect greater ionic (Cu-II-O-2(-1)) character within the valence bond description. A revised interpretation of the physical origins of the O-18 ElEs upon O-2 binding to redox metals is also advanced along with experimental data that should be used as benchmarks for interpreting O-18 kinetic isotope effects upon enzyme reactions.