Electron transfer and 5f→6f transitions in uranium(IV), neptunium(IV), plutonium(IV) hexahalides

Absorption spectra of solutions in acetonitrile of tetra (ethyl) ammonium salts of UCl6 --, UBr6 --, UI6 --, NpCl6 --, NpBr6 --, PuCl6 -- and PuBr6 -- show three types of bands arranged according to increasing half-width: (1) internal 5f q transitions showing a large nephelauxetic effect, the wavenumbers in the hexabromides being some 3–5 per cent smaller than of the corresponding hexachlorides; (2) 5f q →5f q-16d transitions indicating that the 5f→6d orbital energy differences are roughly constant as function of q in contrast to the strong increase in the aqua ions; (3) π 245f q →π 235f q+1 electron transfer bands indicating that the optical electronegativities x opt (corrected for spin-pairing energy and certain effects of L, S, J splitting) are 1·5 for U(IV), 1·75 for Np(IV) and 2·05 for Pu(IV). Further on, the charge separation effects on electron transfer bands are discussed with special regard to uranyl iodide complexes.