INTERACTIONS BETWEEN CATIONS AND SUGARS .7. GIBBS ENERGIES, ENTHALPIES AND ENTROPIES OF ASSOCIATION OF THE TRIVALENT LANTHANIDE CATIONS WITH RIBOSE IN WATER AT 298.15-K

Standard Gibbs energies, enthalpies and entropies of association of th e trivalent lanthanide cations with the complexing sites of ribose hav e been determined in water using a calorimetric method and following a procedure that allows the non-specific interactions to be neglected. For the cations going from La3+ to Tb3+ only 1:1 complexes were observ ed; the stability constants, the values of which range between 3 and 1 1, show a maximum for Sm3+. No significant complexation of the heavier lanthanide cations was detected. The results have been compared with those obtained for stronger ligands also bearing oxygen atoms as coord inating sites, showing that ribose is as selective towards the lanthan ide cations as are these stronger ligands. On the other hand, comparis on of the entropies of the complexes with those of the free cations ha s revealed a behaviour change starting with Eu3+, thus supporting the controversial assumption of a coordination number change along the lan thanide series.