A planar pentacoordinate oxygen in the experimentally observed [Be₅O₆]^2- dianion

Small multiply charged anions (SMCAs) are exceptionally challenging to generate in gas-phase experiments due to the spontaneous detachment of excess electrons. The [Be₅O₆]^2- dianion, first produced in 2006 via electrospray ionization and initially proposed by a concurrent computational study to adopt a linear O-Be-alternating structure, stands as a rare experimentally observed SMCA. In this study, by applying our recently developed electron-compensation strategy, we designed a starlike D _5h [O©Be₅O₅]^2- cluster featuring a planar pentacoordinate oxygen (ppO), which intriguingly shares the molecular formula [Be₅O₆]^2-. Remarkably, this ppO isomer is not only 55.8 kcal mol^-1 more stable than the previously reported linear isomer but also represents the global energy minimum on the [Be₅O₆]^2- potential energy surface. By adhering to the principles of the electron-compensation strategy, all Be atoms in the ppO isomer are electronically compensated and geometrically shielded by peripheral O atoms, resulting in a well-defined electronic structure. This is evidenced by a positive first vertical detachment energy of 2.44 eV, which effectively prevents the spontaneous loss of excess electrons. Thus, our work serendipitously uncovered and elaborately rationalized an experimentally unprecedented ppO within the previously generated SMCA [Be₅O₆]^2-, marking a significant milestone in the field.