To better understand the bonding in complexes of f-elements by polydentate N-donor ligands, the complexation ofamericium(III) and lanthanide(III) cations by 2-amino-4,6-di-(pyridin-2-yl)-1,3,5-triazine (ADPTZ) was studied usinga thermodynamic approach. The stability constants of the 1:1 complexes in a methanol/water mixture (75/25 vol %)were determined by UV-visible spectrophotometry for every lanthanide(III) ion (except promethium), and yttrium(III)and americium(III) cations. The thermodynamic parameters (
H
,
S) of complexation were determined from thetemperature dependence of the stability constants and by microcalorimetry. The trends of the variations of
G,
H, and
S across the lanthanide series are compared with published results for other tridentate ligands andconfirm strongly ionic bonding in the lanthanide-ADPTZ complexes. Comparison of the thermodynamic propertiesbetween the Am- and Ln-ADPTZ complexes highlights an increase in stability of the complexes by a factor of 20in favor of the americium cation. This difference arises from a more exothermic reaction enthalpy in the case ofAm, which is correlated with a greater degree of covalency in the americium-nitrogen bonds. Quantum chemistrycalculations performed on a series of trivalent actinide and lanthanide-ADPTZ complexes support the experimentalresults, showing a slightly greater covalence in the actinide-ligand bonds that originates from a charge transferfrom the ligand
orbitals to the 5f and 6d orbitals of the actinide ion.
