Covalency hinders AnO2(H2O)+ --> AnO(OH)2+ isomerisation (An = Pa-Pu)

The enthalpies of the reactions AnO2 +  AnO+ + O and AnO2 + + H2O  AnO2(H2O)+, and those of the isomerisation of the latter to AnO(OH)2 +, have been calculated for An = Pa–Pu. The data match previous experimental and computational values very closely, and the computed enthalpy for the isomerisation of PaO2(H2O)+ to PaO(OH)2 +, requested by the authors of Inorg. Chem. 2015, 54, 7474, is found to be 0.8 kJ/mol. The NPA, NBO and QTAIM approaches are used to probe covalency in the An–Oyl bond of AnO2(H2O)+, and all metrics agree that these bonds become increasingly covalent as the 5f series is crossed, providing rationalisation for the increasingly endothermic isomerisation reactions. QTAIM analysis indicates that the An=O and An–OH bonds in the oxide hydroxide isomers also become increasingly covalent as the 5f series is crossed.