Bonding trends across the series of tricarbonato-actinyl anions [(AnO2)(CO3)3]4− (An = U–Cm): the plutonium turn
Actinyl-tricarbonato anions [(AnO2)(CO3)3]4− (An = U–Cm) in various environments were investigated using theoretical approaches of quantum-mechanics, molecular-mechanics and cluster-models. Cations and solvent molecules in the 2nd coordination sphere affect the equatorial An←Oeq bonds more than the axial AnOax bonds. Common actinide contraction is found for calculated and experimental axial bond lengths of 92U to 94Pu, though no longer for 94Pu to 96Cm. The tendency of U to Pu forming actinyl(VI) species dwindles away toward Cm, which already features the preferred AnIII/LnIII oxidation state of the later actinides and all lanthanides. The well known change from d-type to typical U–Pu–Cm type and then to f-type behavior is labeled as the plutonium turn, a phenomenon that is caused by f-orbital energy-decrease and f-orbital localization with increase of both nuclear charge and oxidation state, and a non-linear variation of effective f-electron population across the actinide series. Both orbital and configuration mixing and occupation of antibonding 5f type orbitals increase, weakening the AnOax bonds and reducing the highest possible oxidation states of the later actinides.