Thermodynamic and structural trends in hexavalent actinyl cations: complexation of dipicolinic acid with NpO2(2+) and PuO2(2+) in comparison with UO2(2+).

The complexation of NpO2(2+) and PuO2(2+) with dipicolinic acid (DPA) has been investigated in 0.1 M NaClO4 by spectrophotometry, microcalorimetry, and single crystal diffractometry. Formation of 1:1 and 1:2 (metal/ligand molar ratio) complexes of DPA with NpO2(2+) and PuO2(2+) were identified and the thermodynamic parameters were determined and compared with those of UO2(2+) . All three hexavalent actinyl cations form strong 1:1 DPA complexes with slightly decreasing but comparable stability constants from UO2(2+) to PuO2(2+), whereas the stability constants of the 1:2 complexes (logβ2) decrease substantially along the series (16.3 for UO2L2(2-), 15.17 for NpO2L2(2-), and 14.17 for PuO2L2(2-) at 25 °C). The enthalpies of complexation for the 1:2 complexes become less exothermic from UO2L2(2-) (-28.9 kJ mol(-1)), through NpO2L2(2-) (-27.2 kJ mol(-1)), and to PuO2L2(2-) (-22.7 kJ mol(-1)). The trends in the thermodynamic parameters are discussed in terms of the effective charge of the cations and the steric constraints in the structures of the complexes. In addition, the features of the absorption spectra, including the wavelength and intensity of the absorption bands, are related to the perturbation of the ligand field and the symmetry of the actinyl complexes.