Absorption spectra of actinide compounds

From the energy levels of the gaseous ions Ac++, Th+++, and Th++, it can safely be extrapolated that the ground electron configuration of all actinide ions with ionic charges at least + 3 contains only 5f-electrons outside the emanation shells. This is further supported by the 5fn → 5fn-16d transitions observed in Pa(IV), U(III), Np(III) (though their wave numbers are too high to be measured in U(IV) and Np(IV)). The configuration f2 has a very characteristic distribution of energy levels between 3H4 and 3P2 with 1S0 as much higher, isolated level. The absorption spectra of U(IV) complexes and of PuF6 suggest interelectronic repulsion parameters ∼60 per cent of the values in the corresponding lanthanides, while the Landé factors are about twice as large. This would also explain the recent measurements of Cm(III) complexes, compared to Gd(III). The larger average radius of the 5f-shell causes larger perturbation effects from the ligands than in the lanthanides. The J-values will therefore be distributed between several adjacent, observed energy levels without making the general treatment of the fn-configurations impossible. The variation with oxidation numbers in isoelectronic series are discussed, e.g. U(III) and Np(IV); Pu(III) and AmF4; and Am(III) and CmF4. Band positions are predicted in Bk(III) and BkF4. Finally, the chemical properties of the actinide elements are discussed, and it is emphasized that no necessary connections exist between the presence of f-electrons and constant trivalency.