Two new examples of very short thallium–transition metal contacts: Tl3Ag3Sb2S6 and Tl3Ag3As2S6

Two new sulphosalts Tl3Ag3Sb2S6, (1) and Tl3Ag3As2S6, (2) were prepared in reaction of synthetic binary sulfides: argentite (Ag2S), carlinite (Tl2S) and orpiment (As2S3) or stibnite (Sb2S3), and their crystal structures have been determined using single-crystal data. The compounds are isostructural and crystallize in the space group P21/c (Z = 4), with a = 11.6854(17) Å, b = 11.8602(17) Å, c = 12.1294(18) Å, β = 123.356(2)°, V = 1404.1(4) Å3 for 1, and a = 11.4415(14) Å, b = 11.7530(15) Å, c = 11.9880(16) Å, β = 123.516(3)°, V = 1344.0(3) Å3 for 2. Refinements involving anisotropic displacement parameters for all atoms converged to the conventional R-factors: R1 = 0.0496 for 1 and R1 = 0.0372 for 2. The structures consist of two kinds of slabs alternately situated along [1 0 0] direction and sharing S atoms located between them. The first slab is built up of Tl1S8, Tl2S6, Tl3S5Ag3 coordination polyhedra and Ag3S3Tl3 distorted trigonal pyramid (the average 〈Ag3–S〉 distance is 2.542 for 1 and 2.539 Å for 2) combined with trigonal Sb1S3 or As1S3 coordination pyramids (〈Sb1–S〉 and 〈As1–S〉 are 2.438 and 2.259 Å, respectively). The thallium atom Tl1 is surrounded by eight S atoms forming a square anti-prism (〈Tl1–S〉: 3.393 for 1 and 3.356 Å for 2), while the coordination polyhedron of Tl2 can be described as a significantly distorted trigonal prism or as an extremely distorted octahedron (〈Tl2–S〉: 3.236 Å for 1 and 3.238 Å for 2). Thallium Tl3 is surrounded by six atoms (one Ag, four S at shorter and one S at longer distance), which form a distorted trigonal prism (〈Tl3–S〉: 3.165 Å in 1 and 3.155 Å in 2). The second slab consists of trigonal Sb2S3 or As2S3 coordination pyramids combined with Ag1S4 and Ag2S4 coordination tetrahedra (〈Sb2–S〉 and 〈As2–S〉 are 2.433 and 2.251 Å, respectively; 〈Ag1–S〉: 2.623 Å for 1 and 2.612 Å for 2; 〈Ag2–S〉: 2.623 Å for 1 and 2.630 Å for 2). The most interesting feature of both structures is the existence of short Tl3–Ag3 contacts of 2.959(2) Å in 1 and 2.922(1) Å in 2. These contacts are the shortest found so far and indicate very strong Tl–Ag interactions. They are discussed in terms of metallophilicity principle and relativistic effects. It is also pointed out that if only valence shell electrons are considered (Tl–Ag)2+ group is isoelectronic with the (Hg–Hg)2+ ion, therefore new examples of short Tl–Ag contacts could be expected.