The complexation of Ba2+ by noncyclic, macrocyclic and macrobicyclic nitrogen-containing ligands in propylene carbonate

The complex formation between barium(II) and noncyclic diamines, diazacrown ethers and cryptands has been studied in propylene carbonate using potentiometric and calorimetric methods. The ring size, the number of donor atoms and substituents influence the stability constants and thermodynamic values of the complexes formed. From the reaction enthalpies of the diamines with barium(II), an enthalpy of interaction between the cation and one nitrogen donor atom can be calculated. Together with a known value of the enthalpy of interaction with an ether donor atom, the reaction enthalpies of all ligands examined can be predicted. They are at least the upper limits of the values of the reaction enthalpies. Thus, the macrocyclic and macrobicyclic effect is caused by favorable entropic contributions only.