Planar pentacoordinate carbon in a sulphur-surrounded boron wheel: the global minimum of CB 5 S 5 .

We report a σ + π double aromatic CB 5 S 5 + cluster, the first global minimum unusually having a planar hypercoordinate carbon inside a boron wheel. Five peripheral sulfur atoms stabilize the carbon-centered boron wheel by weakening the electron deficiency of the boron atoms through strong S → B π back-bonding.