High oxidation states in actinide chemistry: a DHF investigation of some trends exemplified by the AnO2F4 species

A study of the AnO2F4 species was conducted at the Dirac-Hartree-Fock level for An = Pu, Cm Cf, and Fm, to examine trends in bonding and the possible stability of high-oxidation-state species in the late actinides. The Pu and Cm species were found to be stable at the DHF level, the Cf species was marginally stable and the Fm species unstable to decomposition into the gaseous elements. The latter two species broke the expected D4h symmetry, which could be due to multiconfiguration effects or intrinsic instability. Clear trends of decreasing actinide net charge and 6d population were observed, and evidence for the cost of the 6p hole was also found for Fm. The populations of the spin-orbit components of the 5f shell were found to be highly nonstatistical, with the 5f5/2 filled and relatively inert for Cf and Fm and the 5f7/2 only partially filled and involved in the bonding. This differential filling of the 5f5/2 results in a spin-orbit splitting in the molecule that is larger than in the atom. This behavior suggests that there might still be some possibility of 5f participation in bonding in the late actinides, and therefore of the existence of high-oxidation-state species.