Tl 2 Mo 9 Se 11

The structure of Tl2Mo9Se11, dithallium nonamolybdenum undecaselenide, is isotypic with Tl2Mo9S11 [Potel et al. (1980). Acta Cryst. B36, 1319–1322]. The structural set-up is characterized by a mixture of Mo6Se i 8Se a 6 and Mo12Se i 14Se a 6 cluster units in a 1:1 ratio. Both components are interconnected through interunit Mo—Se bonds. The cluster units are centered at Wyckoff positions 3a and 3b (point-group symmetry 3.). The two Tl atoms are situated in the voids of the three-dimensional arrangement. Two of the five independent Se atoms and the Tl atoms lie on sites with 3. symmetry (Wyckoff site 6c).

The structure of Tl 2 Mo 9 Se 11 , dithallium nonamolybdenum undecaselenide, is isotypic with Tl 2 Mo 9 S 11 [Potel et al. (1980). Acta Cryst. B36, [1319][1320][1321][1322]. The structural set-up is characterized by a mixture of Mo 6 Se i 8 Se a 6 and Mo 12 Se i 14 Se a 6 cluster units in a 1:1 ratio. Both components are interconnected through interunit Mo-Se bonds. The cluster units are centered at Wyckoff positions 3a and 3b (point-group symmetry 3.). The two Tl I atoms are situated in the voids of the three-dimensional arrangement. Two of the five independent Se atoms and the Tl atoms lie on sites with 3. symmetry (Wyckoff site 6c).

Related literature
For the crystal structure of Tl 2 Mo 9 S 11 , see: Potel et al. (1980). For details of the i-and a-type ligand notation, see: Schä fer & von Schnering (1964). Ionic radii were compiled by Shannon (1976). For background to the extinction correction, see: Becker & Coppens (1974).
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: WM2368).
Tl 2 Mo 9 Se 11 P. Gougeon, P. Gall and M. Potel Comment Potel et al. (1980) reported the crystal structure of Tl 2 Mo 9 S 11 that was the first compound containing a transition metal cluster with a nuclearity higher than 9, namely the trioctahedral Mo 12 cluster. The latter cluster, which results from the uniaxial face-sharing of three Mo 6 octahedra, coexists with the octahedral Mo 6 cluster in equal proportion. We present here the crystal structure of the isotypic selenide, Tl 2 Mo 9 Se 11 . The Mo-Se framework of Tl 2 Mo 9 Se 11 consists of an equal mixture of Mo 6 Se i 8 Se a 6 and Mo 12 Se i 14 Se a 6 cluster units interconnected through Mo-Se bonds ( Fig.1 and 2). Details of the i-and a-type ligand notation were reported by Schäfer & von Schnering (1964). The first unit can be described as an atoms, respectively. In Tl 2 Mo 9 S 11 , the corresponding distances are equal to 2.658 (1) and 2.688 (1) Å, respectively. The distances between the triangles formed by the Mo2 and Mo3 atoms are 2.7397 (7) and 2.7901 (7) Å and those between the Mo3 3 triangles, 2.6796 (6) Å. The average Mo-Mo distance within the Mo 12 cluster is similar in Tl 2 Mo 9 S 11 and Tl 2 Mo 9 Se 11 and amounts to 2.705 Å. The Se atoms bridge either one (Se1, Se2, Se4 and Se5) or two (Se3) Mo triangular faces of the clusters. Moreover, the Se1 and Se2 atoms are linked to a Mo atom of a neighboring cluster. The Mo-Se bond distances range from 2.5269 (8)  environment which consists of seven Se atoms as nearest neighbors with Tl-Se distances in the ranges 3.0737 (10) -3.5840 (6) Å and 3.0483 (9) -3.4032 (6) Å for the Tl1 and Tl2 sites, respectively. These seven Se atoms form a monocapped octahedron compressed along the threefold axis.
Three additional Se atoms at 4.3543 (6) and 4.2446 (6) Å from Tl1 and Tl2, respectively, are also observed (Figure 3). The supplementary materials sup-2 average Tl-Se values are 3.72 and 3.54 Å for the Tl1 and Tl2 sites, respectively. Values of 3.57 and 3.68 Å are expected from the sum of the ionic radii for Se 2and Tl + for coordination numbers 8 and 12 (Shannon, 1976).

Experimental
Single crystals of Tl 2 Mo 9 Se 11 were prepared from a mixture of MoSe 2 , TlSe and Mo with the nominal composition Tl 2 Mo 12 Se 14 . Before use, Mo powder was reduced under H 2 flowing gas at 1273 K during ten hours in order to eliminate any trace of oxygen. The binaries MoSe 2 and TlSe were obtained by heating stoichiometric mixtures of the elements in sealed evacuated silica tubes during about 2 days at 1073 and 573 K, respectively. All handlings of materials were done in an argon-filled glove box. The initial mixture (ca 5 g) was cold pressed and loaded into a molybdenum crucible, which was sealed under a low argon pressure using an arc welding system. The charge was heated at the rate of 300 K/h up to 1773 K, the temperature which was held for 48 h, then cooled at 100 K/h down to 1373 K and finally furnace cooled.

Refinement
The highest peak and the deepest hole in the final Fourier map are at 1.67 and 0.83 Å from Tl1 and Se5, respectively.