[Sb 10 Se 10 ] 2+ , a Heteronuclear Polycyclic Polycation from a Room‐Temperature Ionic Liquid

Reaction of antimony, selenium, and selenium(IV) chloride in the Lewis acidic ionic liquid [BMIM]Cl/AlCl 3 (BMIM: 1‐ n ‐butyl‐3‐methylimidazolium) at room temperature yielded air‐sensitive black block‐shaped crystals of [Sb 10 Se 10 ][AlCl 4 ] 2 . The triclinic unit cell (space group ${P\bar 1}$ , a =947.85(2), b =957.79(2), c =1166.31(3) pm; α= 103.622(1), β= 110.318(1), γ =99.868(1)°; Z =1) contains the first mixed antimony/selenium polycation, [Sb 10 Se 10 ] 2+ . The centrosymmetric polycyclic cation consists of two realgar‐like [Sb 4 Se 4 ] cages, which are connected through positively charged, three‐bonded selenium atoms with a central [Sb 2 Se 2 ] ring. Quantum chemical calculations predict semiconducting behavior of the compound and indicate primarily covalent bonding with varying ionic contribution within the [Sb 10 Se 10 ] 2+ polycation, while the interactions between the polycation and the [AlCl 4 ] − anions are predominantly ionic. The applicability of the Zintl concept to the chemical bonding in the heteronuclear polycation was evaluated by a thorough quantum chemical analysis.