Comprehensive mole-ratio, kinetic, EPR and X-ray studies were performed to investigate substitution reactions between square-planar [CuCl₂(en)] (en = ethylenediamine) and square-pyramidal [CuCl₂(terpy)] (terpy = 2,2’:6’,2”-terpyridine) complexes with bio-relevant nucleophiles at 295 K in 0.010 M Tris-HCl buffer (pH = 7.4) in the presence of 0.010 M NaCl. According to mole-ratio and EPR studies, Tris buffer reacts with [CuCl₂(en)], and likely the complex geometry remains square-planar. Kinetic results have shown that the order of reactivity of nucleophiles towards [CuCl₂(en)] in solution for both reaction steps is 1,2,4-triazole > imidazole > pyrazine. The order of reactivity of the azole and diazine ligands toward [CuCl₂(terpy)] for the first reaction step is imidazole > 1,2,4-triazole > pyrazine, while for the second one is pyrazine > 1,2,4-triazole > imidazole. X-ray studies indicate great affinity of ethylenediamine toward Cu(II), which results in crystal structures of [Cu(en)₂]Cl₂·H₂O and [Cu(en)₂][ZnCl₄], instead of dinuclear or heteronuclear complexes with a bridging pyrazine ligand. The recorded EPR spectra demonstrate that all ligands interact with Cu(II) complexes, except in the case of the [CuCl₂(en)] complex, where the competition between ligand and Tris molecules might be observed.