Soft Acid-Base Stabilization of a Planar Pentacoordinate Sulfur in the Dianion [S©Cd5S5]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.