92.
The complexation of NpO
22+ and PuO
22+ with dipicolinic acid (DPA) has been investigated in 0.1 M NaClO
4 by spectrophotometry, microcalorimetry, and single crystal diffractometry. Formation of 1:1 and 1:2 (metal/ligand molar ratio) complexes of DPA with NpO
22+ and PuO
22+ were identified and the thermodynamic parameters were determined and compared with those of UO
22+. All three hexavalent actinyl cations form strong 1:1 DPA complexes with slightly decreasing but comparable stability constants from UO
22+ to PuO
22+, whereas the stability constants of the 1:2 complexes (log
β2) decrease substantially along the series (16.3 for UO
2L
22?, 15.17 for NpO
2L
22?, and 14.17 for PuO
2L
22? at 25 °C). The enthalpies of complexation for the 1:2 complexes become less exothermic from UO
2L
22? (?28.9 kJ mol
?1), through NpO
2L
22? (?27.2 kJ mol
?1), and to PuO
2L
22? (?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.
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