Stable Terbium Oxides (TbO₂ and TbO₄) with an Oxidation State of +IV

The highest experimentally known oxidation state of Tb has long been Tb(IV). Through a comparison of many Kohn-Sham approximations through extended coupled clusters up to the RAS/CASPT2 and iCIPT2 multiconfiguration benchmark methods, a thorough theoretical study of the relative energies of various molecular TbO₂ and TbO₄ isomers with different oxidation states of both Tb and O atoms is presented. The ground state of TbO₂ is an octet, tetravalent terbium(IV) compound with a C_2v-bent structure. Surprisingly, the bent [TbO₂]⁺ cation retains its usual Tb(IV) oxidation state, thus forcing the [O···O] unit into an unusual radical ion (^•O₂^3-). Considering that Tb reacts with four oxygen species to form TbO₄ species, the ground state of TbO₄ has a structure ¹⁰C_2v-[(η²-O₂^-)(TbO₂⁺)] formed by a side-on superoxide coordinating to a bent [TbO₂]⁺, and an energetically competitive isomer, [vdw-(η¹-O₂)TbO₂], has a neutral tetravalent TbO₂ coordinated by an oxygen; these two species have a Tb(IV) oxidation state. Bonding analyses reveal that the Tb 5d orbital significantly contributes to the Tb-O bonding of Tb(IV) compounds and lacks pronounced Tb 4f5d hybridization, which is however crucial for the formation of a high oxidation state of the f-element and is also the reason why Tb cannot be oxidized higher than +IV in TbO₂ and TbO₄.