The Intermetalloid Cluster Cation (CuBi₈ )³

The reaction of Bi, BiCl₃ , and CuCl in the ionic liquid [BMIm]Cl⋅4 AlCl₃ (BMIm=1-n-butyl-3-methylimidazolium) at 180 °C yielded air-sensitive shiny black crystals of (CuBi₈ )[AlCl₄ ]₂ [Al₂ Cl₇ ] and (CuBi₈ )[AlCl₄ ]₃ . For both compounds X-ray diffraction on single crystals revealed monoclinic structures that contain the intermetalloid cluster (CuBi₈ )³⁺ . It is the first pure bismuth cluster with a 3d metal and the first with a metal that does not form binary intermetallics with bismuth under ambient pressure. The cluster can be interpreted either as a copper(I) cation, η⁴ -coordinated by a square-antiprismatic Bi₈²⁺ polycation (Bi-Cu 267 pm), or as a nine-atomic intermetalloid nido-cluster with 22 skeletal electrons and the C_4v symmetry. One of the chloride ions of a tetrahedral [AlCl₄ ]^- group coordinates the copper atom (Cu-Cl 228 pm) and thereby completes its 18 electron count. DFT-based calculations, followed by real-space bonding analysis, revealed a multicenter bonding situation between copper and bismuth atoms with about seven shared electrons.