Planar tetra-, hexa-, octa-, and deca-coordinate Heavier alkaline-earth metals: covalent interactions of d orbitals with carbon chains.

The involvement of d-orbitals and the large atomic radii of heavier alkaline-earth metals (Ae = Ca, Sr, Ba) have significantly expanded the boundaries of planar coordination chemistry, enabling unprecedented configurations up to pentadeca-coordination. However, these same properties present challenges in isolating low-coordinated analogues. Here, we explore the potential energy surfaces of neutral AeCn (n = 4-12) complexes, composed of Ae atoms coordinated by carbon chains, to identify planar hypercoordinate Ae (phAe) species. Our analysis revealed AeCn (n = 4, 6, 8) and BaC10 as the lowest-energy configurations, featuring planar tetra-, hexa-, octa-, and deca-coordinate geometries with C2v symmetry, where bent Cn rings partially enclose the phAe center. In contrast, odd-numbered AeCn complexes (n = 5, 7, 9, 11) emerge only as local minima or distorted phAe structures. Charge transfer from the phAe center to the Cn chains induces polyynic-like character in even-numbered Cn chains, facilitating strong electrostatic interactions. Importantly, significant covalent interactions between the d orbitals of Ae atoms and the σ-delocalized bonds of Cn chains further enhance the thermodynamic stability of phAe, highlighting the fundamental contribution of Ae d-orbitals in the formation of unique planar hypercoordinate bonding motifs.