Low-temperature oxygenation of copper(I) complexes of
N,
N,
N',
N'-tetraethylpropane-1,3-diamine yields solutions containing both
-
2:
2-peroxodicopper(II) (
P) and bis(
-oxo)dicopper(III) (
O) valence isomers. The
P/
O equilibrium position depends on the nature of the counteranion;
P is favored with more basic anions. Titration and EXAFS experiments as well as DFT calculations suggest that axial donation from a sulfonate anion to the copper centers imparts an electronic/electrostatic bias toward the
P isomer.