Heptavalent Neptunium in a Gas-Phase Complex: (Np VII O 3 + )(NO 3 - ) 2 .

A central goal of chemistry is to achieve ultimate oxidation states, including in gas-phase complexes with no condensed phase perturbations. In the case of the actinide elements, the highest established oxidation states are labile Pu(VII) and somewhat more stable Np(VII). We have synthesized and characterized gas-phase AnO ₃ (NO ₃ ) ₂ ^- complexes for An = U, Np, and Pu by endothermic NO ₂ elimination from AnO ₂ (NO ₃ ) ₃ ^- . It was previously demonstrated that the PuO ₃ ⁺ core of PuO ₃ (NO ₃ ) ₂ ^- has a Pu-O ^• radical bond such that the oxidation state is Pu(VI); it follows that in UO ₃ (NO ₃ ) ₂ ^- it is the stable U(VI) oxidation state. On the basis of the relatively more facile synthesis of NpO ₃ (NO ₃ ) ₂ ^- , a Np(VII) oxidation state is inferred. This interpretation is substantiated by reactivity of the three complexes: NO ₂ spontaneously adds to UO ₃ (NO ₃ ) ₂ ^- and PuO ₃ (NO ₃ ) ₂ ^- but not to NpO ₃ (NO ₃ ) ₂ ^- . This unreactive character is attributed to a Np(VII)O ₃ ⁺ core with three stable Np═O bonds, this in contrast to reactive U-O ^• and Pu-O ^• radical bonds. The computed structures and reaction energies for the three AnO ₃ (NO ₃ ) ₂ ^- support the conclusion that the oxidation states are U(VI), Np(VII), and Pu(VI). The results establish the extreme Np(VII) oxidation state in a gas-phase complex, and demonstrate the inherently greater stability of Np(VII) versus Pu(VII).