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Swapping of an oxygen atom of water with that of a pentavalent actinide dioxide cation, AnO 2  + also called an "actinyl", requires activation of an An-O bond. It was previously found that such oxo exchange in the gas phase occurs for the first two actinyls, PaO 2  + and UO 2  + , but not the next two, NpO 2  + and PuO 2  + . The An-O bond dissociation energies (BDEs) decrease from PaO 2  + o PuO 2  + , such that the observation of a parallel decrease in the An-O bond reactivity is intriguing. To elucidate oxo exchange, we here extend experimental studies to AmO 2  + , americyl(V), and CmO 2  + , curyl(V), which were produced in remarkable abundance by electrospray ionization of Am 3+ and Cm 3+ solutions. Like other AnO 2  + , americyl(V) and curyl(V) adsorb up to four H 2 O molecules to form tetrahydrates AnO 2 (H 2 O) 4  + with the actinide hexacoordinated by oxygen atoms. It was found that AmO 2  + does not oxo-exchange, whereas CmO 2  + does, establishing a "turn" to increasing the reactivity from americyl to curyl, which validates computational predictions. Because oxo exchange occurs via conversion of an actinyl(V) hydrate, AnO 2 (H 2 O) + , to an actinide(V) hydroxide, AnO(OH) 2  + , it reflects the propensity for actinyl(V) hydrolysis: PaO 2  + hydrolyzes and oxo-exchanges most easily, despite the fact that it has the highest BDE of all AnO 2  + . A reexamination of the computational results for actinyl(V) oxo exchange reveals distinctive properties and chemistry of curyl(V) species, particularly CmO(OH) 2  + .

Activation of Water by Pentavalent Actinide Dioxide Cations: Characteristic Curium Revealed by a Reactivity Turn after Americium