Oxidation States, Geometries, and Electronic Structures of Plutonium Tetroxide PuO₄ Isomers: Is Octavalent Pu Viable?

Abstract

In neutral chemical compounds, the highest known oxidation state of all elements in the Periodic Table is +VIII. While PuO4 is viewed as an exotic Pu(+VIII) complex, we have shown here that no stable electronic homologue of octavalent RuO4 and OsO4 exists for PuO4, even though Pu has the same number of eight valence electrons as Ru and Os. Using quantum chemical approaches at the levels of quasi-relativistic DFT, MP2, CCSD(T), and CASPT2, we find the ground state of PuO4 as a quintet (5)C2v-(PuO2)(+)(O2)(-) complex with the leading valence configuration of an (f(3))plutonyl(V) unit, loosely coupled to a superoxido (π*(3))O2(-) ligand. This stable isomer is likely detectable as a transient species, while the previously suggested planar (1)D4h-Pu(VIII)O4 isomer is only metastable. Through electronic structure analyses, the bonding and the oxidation states are explained and rationalized. We have predicted the characteristics of the electronic and vibrational spectra to assist future experimental identification of (PuO2)(+)(O2)(-) by IR, UV-vis, and ionization spectroscopy.

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

“…Earlier work also found the square planar NpO 4as the ground-state isomer. 34,35 Bolvin and coworkers 34 attributed the preference of a planar versus a tetrahedral structure to neptunium accepting more electrons from the oxygens via covalent bonding of the 5f-orbitals. The D 4h square planar PuO 4is similar to the neutral PuO 4 structure proposed by Straka et al 36 that was later found to be meta-stable by Huang et al 35 which was later determined to be a meta-stabl As indicated in Table 1, reaction (1) is computed to be exothermic by -174 kJ/mol and -85 kJ/mol for An = Np and Pu, respectively.…”

“…34,35 Bolvin and coworkers 34 attributed the preference of a planar versus a tetrahedral structure to neptunium accepting more electrons from the oxygens via covalent bonding of the 5f-orbitals. The D 4h square planar PuO 4is similar to the neutral PuO 4 structure proposed by Straka et al 36 that was later found to be meta-stable by Huang et al 35 which was later determined to be a meta-stabl As indicated in Table 1, reaction (1) is computed to be exothermic by -174 kJ/mol and -85 kJ/mol for An = Np and Pu, respectively. The net reaction (1) is considered to proceed via addition of O 2 to An(V)O 2 (C 2 O 4 )to yield superoxides An(VI)O 2 (C 2 O 4 )(O 2 ) -, reaction (3), where the nature of the product is based on the spin states and structures, including the O-O distances that are characteristic of superoxide ligands (Supporting Information).…”

On the multi-reference nature of plutonium oxides: PuO22+, PuO2, PuO3 and PuO2(OH)2

“…[11,12] Several density functional theory (DFT) calculations have been performed on plutonium complexes such as oxides in the past. [9,[13][14][15][16][17][18][19] However, albeit DFT is in principle capable of treating open-shell molecules with multi-reference character, [20] it is known that the currently available density functional approximations often perform poorly in such cases. [12,21] It is therefore preferable to employ from the outset multireference approaches such as the Complete-Active-Space Self-Consistent-Field (CASSCF) method and the Density Matrix Renormalization Group (DMRG) algorithm [22][23][24][25][26][27][28][29][30][31] in order to obtain a qualitatively correct zeroth-order wave function for these systems.…”

Synthesis and Structures of Plutonyl Nitrate Complexes: Is Plutonium Heptavalent in PuO3(NO3)2– ?

“…8 However, recent computational studies indicate that PuO 4 as a molecule comprising Pu(VIII) is not stable. 9,10 Instead, PuO 2 F 4 may be a better candidate as a stable species comprising Pu(VIII). 2 In the case of Pu(VII) in the gas phase, it has been predicted that PuO 3 1 which is only ca.…”