Linear relation between the enthalpy of formation and the frequency of the maximum in the electronic absorption spectra of the copper(II)-tetramine complexes

The title relation was obsd. for aq. CuL2+, where L is H2N(CH2)lNH(CH2)mNH(CH2)nNH2 (l, m, n = 2,2,2; 2,3,2; 3,2,3; 3, 3,3), ethylenediamine, and 1,3-propanediamine, and is discussed in terms of calcg. heats of formation of tetracoordinated Cu(II) complexes with macrocyclic ligands for which calorimetric detns. are difficult. The term macrocyclic effect was introduced to emphasize the exceptional stability of such complexes. Unlike the chelate effect which is essentially and entropy effect, both entropy and enthalpy terms contribute to the macrocyclic effect.