The Mixed-Valence Catena-Heteropolycation (Bi₂S₂)⁺

The reaction of Bi and Bi₂S₃ in the Lewis-acidic ionic liquid [BMIm]Cl ⋅ 4.3AlCl₃ (BMIm=1-n-butyl-3-methylimidazolium) at 200 °C yielded air-sensitive black crystals of (Bi₂S₂)[AlCl₄]. X-ray diffraction on single-crystals revealed a monoclinic structure containing infinite chains ∞¹(Bi₂S₂)⁺. These polycations contain bismuth(II,III) and sulfur(-II) atoms, connected by polar covalent Bi−S and Bi−Bi bonds. The assignment of bonds is ambiguous as a distinction between primary and secondary interactions is arbitrary based on interatomic distances. A real-space quantum chemical bonding analysis, including delocalization indices, helped to quantify the interactions and charge distribution, confirming that a simple Lewis formula is inappropriate to describe the polycation.