In the case of the free U3+ ion, the 5f 2 6d1 level is over 30 000 cm−1 above the5f 3 ground state, while in U3+(aq) the charge-transfer transitions start around 24 000 cm−1 ; solvation thus has a very significant effect upon the relative energies of the 5f and 6d electrons. Metal–ligand charge-transfer transitions have their maxima out in the ultraviolet, but, as with transition metals, the tail of these broad and often very intense absorption bands runs into the visible region of the spectrum, and is responsible for the red, brown, or yellow colours often noted for actinide complexes with polarizable ligands like Br or I.
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