Beryllium-beryllium bonding. 2. Stability of polyatomic dications Be2Hn 2+, for n = 1-4

This ab initio study on the equilibrium geometries and stabilities of the title dications indicates that on the ground surface at least two (Be2H2+) or three (Be2H2 2+, Be2H3 2+) conformations are metastable (quasi-bound). Metastability is essentially governed by the stability of the BeBe bond since abstractions of H or H2 are endothermic reactions. Be2H2+ is bent and 0.5 eV more stable than the linear isomer. For Be2H2 2+, bridged BeH2Be2+ and T-shaped BeBeH2 2+ are species having comparable stabilities, whereas linear HBeBeH2+ is about 4.0 eV less stable. For Be2H3 2+, doubly-bridged HBeH2Be2+ and T-shaped HBeBeH2 2+ also have similar stabilities, whereas triply-bridged BeH3Be2+ lies 1.7 eV higher. For Be2H4 2+, the singlet state of H2BeBeH2 2+ is preferred over the triplet state of bridged HBeH2BeH2+ by 4.2 eV. Hydrogenation of Be2 2+ stabilizes the metal-metal bond relative to 2Be+ + nH2 because of charge delocalization. Bridged BeHnBe2+ isomers also owe their stability to occupation of the strongly-bound πu MOs of Be-Be by the bridging hydrogens. Despite these stabilizing effects, higher hydrides still remain unstable, as exemplified by the reactions Be2H4 2+ → BeH3 + + BeH+ (ΔH = -2 30 eV) and Be2H8 2+ → 2BeH3 + + H2 (ΔH = -3.09 eV)