Is Octavalent Pu(VIII) Possible? Mapping the Plutonium Oxyfluoride Series PuO_<i>n</i>F_{8–2<i>n</i>} (<i>n</i> = 0–4)

While the oxidation state Pu­(VIII) is shown to be less stable than Pu­(V) in the PuO₄ molecule, it is not clear if the more electronegative fluorine can help to stabilize Pu­(VIII). Our calculations on PuO_<i>n</i>F_{8–2<i>n</i>} (<i>n</i> = 0–4) molecules notably confirm that PuO₂F₄ has both ¹<i>D</i>_4<i>h</i> and ⁵<i>C</i>_2<i>v</i> minima with the oxidation states Pu­(VIII) and Pu­(V), respectively, with the latter having lower energy. The hybrid-DFT, CCSD­(T), and CASSCF methods all give the same result. The results conform to a superoxide ligand when <i>n</i> ≥ 2. PuF₈ in a ¹<i>O</i>_<i>h</i> state can decompose to PuF₆ and F₂, and PuOF₆ in a ¹<i>C</i>_2<i>v</i> state also can break down to PuF₆ and 1/2 O₂. The Pu­(VIII) anion PuO₂F₅^– does have a <i>D</i>_5<i>h</i> minimum, which also lies above a ⁵<i>C</i>_2<i>v</i> Pu­(V) peroxide structure. However, the energy differences between the different minima are not large, indicating that metastable species with oxidation states higher than Pu­(V) cannot be completely excluded