RbSb2 – Eine mit KSb2 verwandte Zintl‐Phase

RbSb₂ – A Zintl Phase related to KSb₂ The electron‐precise Zintl compound RbSb₂, which was known to melt incongruently at 418 °C, has been prepared in pure phase from elemental rubidium and antimony in sealed tantalum crucibles. In accordance with the ribbon‐shaped antimonide anions, the compound crystallizes with extremely thin intergrown, mechanically and chemically very sensitive needles of dark‐metallic lustre. The crystal structure could be determined and refined using single crystal x‐ray data (monoclinic, space group C2/m, a = 1403(2), b = 414.0(4), c = 855.7(14) pm, β = 104.45(12)°, Z = 4, R1 = 0.0901) despite the poor quality of the crystals. It shows fused six‐membered rings of two‐ and three‐bonded Sb atoms forming ribbons running along the monoclinic b axis, which can be interpreted as sections of the elemental structure of antimony (d_Sb‐Sb = 281.9(5) and 286.0(9) pm respectively). The structure of RbSb₂ is thus closely related to that of KSb₂, which exhibits identical antimony anions. Compared to the potassium compound, the ribbons are reoriented against each so that the coordination number of the A counter ions is increased from 6 + 2 (for A = K) to 8 + 2 (for A = Rb). The results of a FP‐LAPW band structure calculation of RbSb₂ are used to explain the chemical bonding in this classical Zintl phase with a calculated indirect band gap of 0.38 eV.