Large 2π aromatic six-membered Al/In rings stabilized by transition-metal-like heavy alkaline earth metals

Density functional theory calculations revealed that four half-sandwich Ae(MH)₆ (Ae = Sr or Ba; M = Al or In) complexes all feature large six-membered Al₆/In₆ rings with rare 2π aromaticity. Together with two Ca analogues, these six molecules all exhibit good thermodynamic and kinetic stabilities. Heavy Ca/Sr/Ba atoms can effectively stabilize the two hexagonal rings via both ionic and covalent interactions involving their ns, (n − 1)p and even (n − 1)d orbitals. Our work not only adds new members to the emerging 2π-aromatic ring family, but also demonstrates the critical role of the transition-metal-like behavior of Ca/Sr/Ba in stabilizing novel metalloaromatic ring structures.