Relation between electronic spectra and heat of formation of some copper-polyamine complexes and the macrocyclic effect

The previously described relationship between ΔHaq and ν(d-d) for a series of tetragonal Cu(II) complexes was extended to include the ammonia complexes Cu(NH3)x(aq)2+ (x = 1 to 4), various monodiamine derivs., some triamine complexes, and a macrocyclic deriv. In the light of the more extensive study, the significance of the relationship is reassessed. The good behavior of the copper complex with the macrocycle cyclen suggests that the "macrocyclic effect" is not due to an increased enthalpy contribution but rather to an increased entropy term. At least for the complexes studied here the difference in solvation enthalpy between open-chain and macrocyclic ligands does not seem to play an important role. This conclusion is also supported by data concerning the enthalpy of dehydration of hydrated open-chain and macrocyclic ligands. The strength of the copper-nitrogen bond in macrocyclic complexes may be greater than or less than the strength in analogous open-chain amine complexes depending upon details of the structure, in particular ring sizes.