Soft Acid–Base Stabilization of a Planar Pentacoordinate Sulfur in the Dianion [S©Cd ₅ S ₅ ] ²–

Group 12 metals exhibit a strong affinity for sulfur, suggesting that soft acid-soft base combinations could provide a viable platform for stabilizing nonclassical planar hypercoordinate motifs. Here, we show that this strategy successfully yields a robust planar pentacoordinate sulfur (ppS) center in the dianion [S©Cd₅S₅]²⁻. In contrast, the Zn and Hg congeners fail to form minima with D5h symmetry due to geometric incompatibility. The Zn₅S₅ ring is too compact, whereas Hg₅S₅ is overly expanded to support a central sulfur atom. The Cd analogue achieves the optimal balance between size and softness, resulting in a true global minimum lying 24.1 kcal mol⁻ ¹ below the nearest isomer and retaining structural integrity in molecular dynamics simulations up to 1000 K. This dianion displays key indicators of experimental viability, including a positive vertical detachment energy (1.90 eV) and a large HOMO-LUMO gap (5.02 eV), which surpass those of previously reported planar pentacoordinate oxygen and sulfur systems. Energy-decomposition analysis reveals that electrostatic attraction is the dominant stabilizing interaction, complemented by moderate covalent contributions arising from an electron-compensation mechanism: five Cd-S-Cd three-center π back-bonds and three 6c-2e σ-bonds anchoring the ppS center to the Cd₅S₅ ring. Magnetic response analyses confirm the absence of global aromaticity, indicating that stability originates from electrostatic interactions rather than delocalized ring currents. Together, these results establish [S©Cd₅S₅]²⁻ as a chemically viable cluster for realizing planar pentacoordinate sulfur in the gas phase and illustrate a generalizable strategy for stabilizing soft-base hypercoordinate centers.