Heptavalent Actinide Tetroxides NpO₄^– and PuO₄^–: Oxidation of Pu(V) to Pu(VII) by Adding an Electron to PuO₄

Abstract

The highest known actinide oxidation states are Np(VII) and Pu(VII), both of which have been identified in solution and solid compounds. Recently a molecular Np(VII) complex, NpO(NO), was prepared and characterized in the gas phase. In accord with the lower stability of heptavalent Pu, no Pu(VII) molecular species has been identified. Reported here are the gas-phase syntheses and characterizations of NpO and PuO. Reactivity studies and density functional theory computations indicate the heptavalent metal oxidation state in both. This is the first instance of Pu(VII) in the absence of stabilizing effects due to condensed phase solvation or crystal fields. The results indicate that addition of an electron to neutral PuO, which has a computed electron affinity of 2.56 eV, counterintuitively results in oxidation of Pu(V) to Pu(VII), concomitant with superoxide reduction.

Probing the Electronic Structure and Chemical Bonding of Uranium Nitride Complexes of NU–XO (X = C, N, O)

“…Thus, the NU–OO complex can be described as [(NU) 2+ (O 2 ) 2– ], a neutral uranium nitride-peroxide species with U in the oxidation state +V. The structure and bonding of [(NU) 2+ (O 2 ) 2– ] are reminiscent of those of actinide tetraoxides (UO 4 and PuO 4 ), , which were computed to be [(An +V O 2 ) + (O 2 ) − ] species as well. As shown in Figure , the bonding between NU and O 2 is dominated by the orbital interaction between the empty U 5f π orbital of UN and the antibonding π* orbitals of O 2 , which results in an increased U 5f π orbital energy and decreased π* orbital energy.…”

On the highest oxidation states of the actinoids in AnO4 molecules (An = Ac – Cm): A DMRG‐CASSCF study

“…There is a continuous interest in synthesizing new compounds with highest OSs, of metallic elements in particular, including the lanthanoids and actinoids 10–25 . Previous experimental and theoretical studies have shown that the highest OSs of the 3d, 4d, 5d (and 4f) elements exhibit “volcano”‐type variations, what had already been noted by Langmuir in 1919 26 .…”

“…Gibson et al recently showed that upon reducing Pu V O 4 , Pu is oxidized to form the Pu VII O 4 À ion. 23 Zaitsevskii and co-workers performed relativistic DFT calculations on AnO 4 (An = Pu, Am, and Cm) molecules and found that the ground states have dioxide-superoxide-like structures [AnO 2 ](O 2 ) with all three actinoids in OS = V. 56 Further theoretical work on AnO 3 , AnO 4 , An 2 O n (An = Pu, Am) and on PuAmO n found the most stable form as PuAmO 4 . 57 However, because of the complexity of electron correlation and spin-orbit coupling in the Pu, Am, and Cm containing molecules, systematic investigations on the OS assignment of actinoid elements in AnO 4 appear still necessary at higher levels of sophistication.…”

Electronic Structures and Unusual Chemical Bonding in Actinyl Peroxide Dimers [An2O6]2+ and [(An2O6)(12-crown-4 ether)2]2+ (An = U, Np, and Pu)

“…Compared to the flourishing uranyl peroxides, their transuranic counterparts are still in infancy, despite their strategic importance in the nuclear industry. Specifically, neptunium (Np) and plutonium (Pu) are two of the most studied transuranic elements, reported to form various compounds with oxide, sulfate, phosphate, arsenate, and other oxyanions, − However, the Np and Pu actinyl peroxide chemistry is largely unexplored. There are only one structurally characterized neptunyl peroxide polymeric compounds with a Np 24 unit, some mononeptunyl peroxide complexes, such as Ca 2 [NpO 2 (O 2 ) 3 ]·9H 2 O, and [NpO 2 (O 2 ) 3 ] polyhedra .…”