A planar pentacoordinate oxygen in the experimentally observed [BeO] dianion.

Small multiply charged anions (SMCAs) are exceptionally challenging to generate in gas-phase experiments due to the spontaneous detachment of excess electrons. The [BeO] dianion, first produced in 2006 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 [O©BeO] cluster featuring a planar pentacoordinate oxygen (ppO), which intriguingly shares the molecular formula [BeO]. Remarkably, this ppO isomer is not only 55.8 kcal mol more stable than the previously reported linear isomer but also represents the global energy minimum on the [BeO] 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 [BeO], marking a significant milestone in the field.