Star-like Cluster S₆Mg₅^2-: A Binary Dianion Global Minimum Featuring a Planar Pentacoordinate Sulfur

For over half a century, clusters exhibiting unconventional bonding have captivated researchers due to their unique electronic characteristics. While most elements in the periodic table demonstrate this remarkable structural feature, sulfur has been notably absent from known global minima with a planar pentacoordinate center. Herein, we report the first binary dianion cluster, S₆Mg₅^2-, featuring a planar pentacoordinate sulfur (ppS) atom. High-level ab initio calculations reveal that its global minimum adopts a singlet state with D_5h symmetry. Bonding analyses indicate that the interactions between the central ppS atom and the peripheral Mg₅S₅ framework are dominated by electrostatic forces. Born-Oppenheimer molecular dynamics (BOMD) simulations demonstrate the cluster's high molecular rigidity, showing dynamic stability up to 1000 K. A significant HOMO-LUMO gap of 4.62 eV and a positive vertical detachment energy (VDE) of 2.49 eV highlight its robust electronic structure. These findings challenge conventional bonding paradigms, deepen the fundamental understanding of ppS-containing clusters, and significantly advance the field of planar hypercoordination chemistry.