Abstract Bismacrocyclic Ni(II)Ni(II) and Cu(II)Ni(II) complexes, i.e., 7,7′-polymethylenebis[2,12-dimethyl-3,7,11,17-tetraazabicyclo[1 1.3.1]heptadeca-1(17),2,11,13,15-pentaene]dinickel(II) perchlorates and 7,7′-polymethylenebis[2,12-dimethyl-3,7,11,17-tetraazabicyclo[11.3.1]heptadeca-l(17),2,11,13,15-pentaene]copper(II)nickel(II) perchlorates with polymethylene bridges 2–6-C long, were found to exist in an equilibrium between square planar and octahedral Ni(II) ions in water. The equilibrium constants and the thermodynamic parameters, ΔH and ΔS, for the Cu(II)Ni(II) complexes were evaluated on the basis of the analyses of the temperature-dependent absorption spectra. The formation of octahedral species was found to be exothermic; this reflects the predominant contribution of an exothermic change to the formation of Ni(II)–water bonds. The absolute values of ΔS corresponding to those expected for the liberation of two molecules of water are not in conflict with the equilibrium proposed. For both the Ni(II)Ni(II) and Cu(II)Ni(II) complexes, the equilibrium shifts toward the octahedral side as the polymethylene bridge is shortened.