Dinuclear [Cu-2{mu-O)(2)(Tp(R,R'))(2)] complexes, analogues of the active site of oxyhemocyanin, are theoretically studied, and the effect of the substituents of the tris(pyrazolyl)borate ligands, Tp(R,R') is analyzed. Density functional theory calculations reveal that the type of bridging oxygen, peroxo, or bisoxo is strongly influenced by the nature and position of the R substituents because of variable substituent... bridging oxygen interactions, as well as electronic effects. The electronic effects of ligands at the 5 position are not significant, but peroxo complexes are favored by electron-withdrawing groups at the 3 position while bisoxo ones are strongly sterically disfavored.