Bonding trends across the series of tricarbonato-actinyl anions [(AnO2)(CO3)3]4- (An = U-Cm): the plutonium turn

Actinyl-tricarbonato anions [(AnO(2))(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 <- O-eq bonds more than the axial An = O-ax bonds. Common actinide contraction is found for calculated and experimental axial bond lengths of U-92 to Pu-94, though no longer for Pu-94 to Cm-96. The tendency of U to Pu forming actinyl(VI) species dwindles away toward Cm, which already features the preferred An(III)/Ln(III) 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 AnO(ax) bonds and reducing the highest possible oxidation states of the later actinides.