Beryllole is the first alkaline earth metal antiaromatic molecule

The heterocyclic beryllole is the first antiaromatic molecule containing an alkaline earth metal in its π system.¹

While metals of the p, d and f block of the periodic table have all been shown to engage in π systems, there has never before been any aromatic or antiaromatic compounds containing s-block elements. Alkaline earth metals had been considered simply too inflexible in terms of their oxidation states to allow them to participate in π bonding. However, a few promising candidates for the first s-block π system emerged within the last decade.