Lawrence Berkeley National Laboratory

Actinyl chemistry is extended beyond Cm to BkO₂⁺ and CfO₂⁺ through transfer of an O atom from NO₂ to BkO⁺ or CfO⁺ , establishing a surprisingly high lower limit of 73 kcal mol^-1 for the dissociation energies, D[O-(BkO⁺ )] and D[O-(CfO⁺ )]. CCSD(T) computations are in accord with the observed reactions, and characterize the newly observed dioxide ions as linear pentavalent actinyls; these being the first Bk and Cf species with oxidation states above IV. Computations of actinide dioxide cations AnO₂⁺ for An=Pa to Lr reveal an unexpected minimum for D[O-(CmO⁺ )]. For CmO₂⁺ , and AnO₂⁺ beyond EsO₂⁺ , the most stable structure has side-on bonded η² -(O₂ ), as An^III peroxides for An=Cm and Lr, and as An^II superoxides for An=Fm, Md, and No. It is predicted that the most stable structure of EsO₂⁺ is linear [O=Es^V =O]⁺ , einsteinyl, and that FmO₂⁺ and MdO₂⁺ , like CmO₂⁺ , also have actinyl(V) structures as local energy minima. The results expand actinide oxidation state chemistry, the realm of the distinctive actinyl moiety, and the non-periodic character towards the end of the periodic table.