[S©Cd₅S₅]^2-: a planar pentacoordinate sulfur dianion stabilized by a transition-metal framework

While planar hypercoordinate clusters centered on electronegative atoms have been reported, analogous architectures incorporating transition-metal ligands remain scarcely explored. Herein, twelve [Y©M₅Y₅]^2- clusters (M = Zn, Cd, Hg; Y = O, S, Se, Te) adopting D_5h starlike geometries were designed. The planar pentacoordinate sulfur (ppS) species [S©Cd₅S₅]^2- was identified as the global minimum, lying 24.7 kcal mol^-1 below the next isomer at the CCSD(T) level. Born-Oppenheimer molecular dynamics simulations (up to 600 K) confirmed its exceptional kinetic stability. Bonding analyses revealed pronounced Cd → S charge transfer and S → Cd π-backdonation, leading to strong electrostatic and weak covalent interactions that jointly stabilize the Cd₅S₅ framework and the central ppS motif. These results identify [S©Cd₅S₅]^2- as a highly stable ppS dianion, providing a promising target for experimental realization and broadening the scope of planar hypercoordinate sulfur chemistry.