A spectrophotometric study of trivalent actinide complexes in solution-II. Neptunium and plutonium

Spectra of neptunium (III) and plutonium (III) were studied in aqueous halide solutions as functions of the halide concentration. Each element was found to exhibit strong absorption at a characteristic wavelength in concentrated lithium chloride or bromide solutions. In chloride solutions neptunium and plutonium have intense absorption bands at 26,050 and 32,050 cm^-1 respectively, and in bromide solutions the bands are shifted about 250 cm^-1 towards lower energies. These bands are attributed to 5fⁿ → 5f^n-1 6d¹ transitions. The stability of the complexes MX₂⁺ could be estimated, being the same, within experimental error for both M = Np and M = Pu, the values for the effective constants β₂^* being (1·0 ± 0·1) × 10^-5 M^-2 for X = Cl and (3·1 ± 0·4) × 10^-7 M^-2 for X = Br. At intermediate halide concentrations, the species MX²⁺ are formed. The stability of these complexes are much lower than those of solvent shared ion pairs of these elements, as is the case with uranium (III).