Enthalpies of Formation of the Monofluorolanthanide Complexes

The enthalpies of formation of the monofluoro complexes of 13 lanthanide elements and yttrium have been measured in aqueous solution at 25/degree/C and an ionic strength of 1.0 M by titration calorimetry. Their results for YF/sup 2+/ compare well with previously reported determinations obtained from temperature-coefficient variations of potentiometric data. The enthalpies of complexation for LnF/sup 2+/ were found to be endothermic for all lanthanide metals studied and show considerably less variation with increasing atomic number than previous studies. The authors values for /Delta/H/sub 101/ for the lanthanides are much less endothermic than the earlier determinations and appear to agree well with electrostatic models which correlate entropies for fluoride complex formation with charge/radius ratios. These results indicate that the coordination behavior of the fluoride ion is more similar to that of the acetate ion than was previously though. 17 refs., 3 figs., 1 tab.