Oxygen activation by copper(I) complexes with tetra- or pentadentate mono- or dinucleating bispidine ligands is known to lead to unusually stable end-on-[{(bispicline)Cu}(2)(O-2)](2+) complexes (bispidines are methyl-2,4-bis(2-pyridin-yl)-3,7-diazabicyclo-[3.3.1]-nonane-9-diol-1,5-dicarboxylates); catecholase activity of these dinuclear Cu-II/I systems has been demonstrated experimentally, and the mechanism has been thoroughly analyzed: The present density functional theory (DFT) based study provides an analysis of the electronic structure and catalytic activity of [{(bispidine)Cu}(2)(O-2)](2+). As a result of the unique square pyramidal coordination geometry, the d(x2-y2) ground state leads to an unusual sigma/pi bonding pattern, responsible for the stability of the peroxo complex and the observed catecholase activity with a unique mechanistic pathway. The oxidation of catechol to ortho-quinone (one molecule per catalytic cycle and concomitant formation of one equivalent of H2O2) is shown to occur via an associative, stepwise pathway. The unusual stability of the end-on-, peroxo-dicopper(II) complex and isomerization to copper(II) complexes with chelating catecholate ligands, which inhibit the catalytic Fide, are shown to be responsible for an only moderate catalytic activity