Reactions of the Rb-Tl-As system near 500 degrees 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) Angstrom, b = 11.104(3) Angstrom, c = 7.894(4) Angstrom. 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 farmer. EHMO calculations for TaAs47- (T-d) and TaAs4Tl25- (D-2h) 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 to) and sigma plus pi(\\) in the plane of the TlAs2Ta portions and 5c-4e d-p bonds within TaAs4.