The oxidation state is one of the most important concepts in chemistry [1]. As an indication of the electronic state of atoms in a molecule, the "oxidation state" of an atom represents the number of valence electrons it has formally gained or lost when bonded with other atoms in a molecule [2]. The highest known oxidation state is +8, as exemplified in tetra-oxides MO4 (M = Ru, Os, Xe) of group-VIII metal and less-inert noble gas. Despite claims of octavalent states of Pu and Cm in MO4 compounds, whether or not such high oxidation state is stable for actinides is still unclear. We have investigated the electronic structures of PuO4 and analogous MO4 (M = Fe, Ru, Os, Hs, Sm) complexes by using ab initio wavefunction theory and density functional theory [3,4]. It has been shown that the exotic low-spin high-oxidation state of Pu(VIII) is highly unstable against intramolecular electron transfer. The more common plutonyl(V)+ unit may be loosely coupled to a superoxido O2− ligand, forming PuO4 as a high-spin complex of lower oxidation state, 5C2v-(PuO2)+(O2)−. The leading valence configuration with open electron shells on plutonium, (f3)Pu5+, and superoxide dioxygen, (π*3)O2−, yields a highly correlated valence electronic system. We can thus conclude that the highest oxidation states of the actinides become more and more unlikely from uranium onward in the 5f actinoid series. References [1] P. Karen, P. McArdle, J. Takats, “Towards a Comprehensive Definition of Oxidation State (IUPAC Technical Report)”, Pure Appl. Chem. 2014, 86, 1017-1081. [2] H. Xiao, H.-S. Hu, W. H. E. Schwarz, J. Li, "Theoretical Investigations of Geometry, Electronic Structure and Stability of UO6: Octahedral Uranium Hexoxide and Its Isomers", J. Phys. Chem. A 2010, 114, 8837−8844. [3] W. Huang, W.-H. Xu, J. Su, W. H. E. Schwarz, J. Li, "Oxidation States, Geometries, and Electronic Structures of Plutonium Tetroxide PuO4 Isomers: Is Octavalent Pu Viable?", Inorg. Chem. 2013, 52, 14237-14245. [4] W. Huang, W. H. E. Schwarz, J. Li, "Periodic Trend in Metal Tetroxide MO4 (M = Fe, Ru, Os, Hs, Sm, Pu)", manuscript submitted.

Exploring the highest oxidation state in actinide compounds

249th National Meeting (2015)

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