C 4 Ae (Ae = Be–Ba) Cluster: Planar Tetracoordinate Alkaline Earth Metals and Involvement of d‐Orbitals for Heavier Alkaline Earth Metals

ABSTRACT Planar hypercoordination in s‐block metals involving covalent bonding is a rare phenomenon. Herein, we report 18 valence‐electron‐neutral C 4 Ae (Ae = Be, Mg, Ca, Sr, Ba) clusters based on density functional and ab initio methods, which feature planar tetracoordinate alkaline earth metals (ptAe) in C 2V symmetric global minimum geometries. The global minima C 2V geometries are well defined in the potential energy surface, that is, the closest minimum lies significantly higher in energy. All the Ae‐C bonds are dominated by electrostatic contribution with non‐negligible covalent character. Involvement of d‐orbital participation of heavier alkaline‐earth metals (Ca, Sr, Ba) in covalent bonding justifies them as “honorary transition metals”. Conceptual density functional theory (CDFT) predicts that ptBe has the highest hardness and is chemically less reactive.