A(5)TaAs(4)Tl(2) (A = Rb, K). Transition-Metal Zintl Phases with a Novel Complex Ion: Synthesis, Structure, and Bonding.
Reactions of the Rb-Tl-As system near 500 degrees C in tantalum containers always lead to the formation of Rb(5)TaAs(4)Tl(2) because of reactions with the container wall. On-stoichiometry reactions of these three elements in Ta, or of the four components in SiO(2), afford the pure compound. Rb(5)TaAs(4)Tl(2) 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 K(5)TaAs(4)Tl(2) also exists. The main structural feature is the closed-shell polyanion TaAs(4)Tl(2)(5)(-) which can be viewed as the result of a reaction of a TaAs(4)(7)(-) tetrahedron with two Tl(+) so as to bridge opposed edges of the former. EHMO calculations for TaAs(4)(7)(-) (T(d)()) and TaAs(4)Tl(2)(5)(-) (D(2)(h)()) demonstrate the character of the bonding of Tl s and p to the electron-rich As. Two kinds of multicentered bonds are evident: pi( perpendicular) and sigma plus pi( parallel) in the plane of the TlAs(2)Ta portions and 5c-4e d-p bonds within TaAs(4).
Huang
DaPing
D
Ames Laboratory-DOE(1) and Department of Chemistry, Iowa State University, Ames, Iowa 50011.
Corbett
John D.
JD