A5TaAs4Tl2 (A = Rb, K). Transition-Metal Zintl Phases with a Novel Complex Ion: Synthesis, Structure, and Bonding
journal contributionposted on 24.07.1998 by DaPing Huang, John D. Corbett
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Reactions of the Rb−Tl−As system near 500 °C in tantalum containers always lead to the formation of Rb5TaAs4Tl2 because of reactions with the container wall. On-stoichiometry reactions of these three elements in Ta, or of the four components in SiO2, afford the pure compound. Rb5TaAs4Tl2 crystallizes in orthorhombic system, space group Pnma (No. 62), Z = 4, a = 19.196(5) Å, b = 11.104(3) Å, c = 7.894(4) Å. The isostructural K5TaAs4Tl2 also exists. The main structural feature is the closed-shell polyanion TaAs4Tl25- which can be viewed as the result of a reaction of a TaAs47- tetrahedron with two Tl+ so as to bridge opposed edges of the former. EHMO calculations for TaAs47- (Td) and TaAs4Tl25- (D2h) demonstrate the character of the bonding of Tl s and p to the electron-rich As. Two kinds of multicentered bonds are evident: π⊥ and σ plus π∥ in the plane of the TlAs2Ta portions and 5c−4e d−p bonds within TaAs4.