A novel unsymmetric dinucleating ligand (L-N3N4) combining a tridentate and a tetradentate binding sites linked through a m-xylyl spacer was synthesized as ligand scaffold for preparing homo- and dimetallic complexes, where the two metal ions are bound in two different coordination environments. Site-selective binding of different metal ions is demonstrated. L-N3N4 is able to discriminate between Cu-I and a complementary metal (M' = Cu-I, Zn-II, Fe-II, Cu-II, or Ga-III) so that pure heterodimetallic complexes with a general formula [(CuM)-M-I'(L-N3N4)](n+) are synthesized. Reaction of the dicopper(I) complex [Cu-2(I)(L-N3N4)](2+) with O-2 leads to the formation of two different copper-dioxygen (Cu2O2) intermolecular species (O and P-T) between two copper atoms located in the same site from different complex molecules. Taking advantage of this feature, reaction of the heterodimetallic complexes [CuM'(L-N3N4)](n+) with O-2 at low temperature is used as a tool to determine the final position of the CuI center in the system because only one of the two Cu2O2 species is formed.