No abstract

RbNb 4 Br 11 was obtained by a solid state reaction of niobium powder, NbBr 5 and RbBr in a sealed niobium tube at 500°C. It crystallizes in the orthorhombic space group Pmma with a = 1699.5(1), b = 726.7(1), c = 694.5(1) pm, V = 857.6(1)·10 6 pm 3 , Z = 2 and is isotypic with CsNb 4 Br 11 . Four niobium atoms build a cluster in the shape of a rhombus with bonding distances of 295 pm (Nb1-Nb2) and 304 pm (Nb2-Nb2), respectively. The structure consists of [{Nb 4 }Br 6 i Br 10/2 a-a ] -

A facile route to single crystals of Nb 3 Br 8 by the reaction of NbBr 5 with the wall of the niobium reaction vessel at 800 • C is reported. The crystal structure (a = 707.87(5), c = 3895.7(6) pm, R3m, Z = 6) was determined from diffractometer data; the crystal data are compared with those obtained from film data in 1966 which were of already high precision. Triangular {Nb 3 } clusters with Nb-Nb distances of 288.6(2) pm are surrounded by 13 bromide ligands which bridge them to double layers, {Nb 3 }(µ 3 -Br4) i 1/1 (µ 2 -Br1) i (3/2)·2 (µ 1 -Br2) a-a 6/2 (µ 1 -Br3) a-a-a 3/3 that are stacked to a 12R structure.

Well ordered single crystals of TaI 5 (orthorhombic, Pnma (no. 62), a ϭ 1396.2(2), b ϭ 2009.5(4), c ϭ 660.7(1) pm, Z ϭ 8, R 1 for 967 reflections with I 0 >2σ(I 0 ): 0.0381) were obtained from a reaction of the elements. The structure determination confirms Müller's prediction of 1978. Single crystals of CsTaI 6 (monoclinic, C2/c (no. 15), a ϭ 1362.3(2), b ϭ 709.9(1), c ϭ 1736.2(3) pm, β ϭ 128.77(1)°, Z ϭ 4, R 1 for 1140 reflections with I 0 >2σ(I 0 ): 0.0378) The system tantalum/iodine contains Ϫ as one might have expected Ϫ as the most oxidized compound tantalum pentaiodide, TaI 5 [1].The structure was previously determined from a one-dimensionally disordered crystal [2]. It contains dimers of edge-connected octahedra, Ta 2 I 10 . Ternary iodides ATaI 6 (with A representing an alkali-metal ion) have apparently not been investigated previously. We have now obtained well ordered single crystals of TaI 5 and of the first ternary iodide of tantalum(V), CsTaI 6 [3].

Key indicators: single-crystal X-ray study; T = 293 K; mean (Ta-Br) = 0.002 Å; R factor = 0.042; wR factor = 0.109; data-to-parameter ratio = 29.6.Crystals of di--bromido-bis[tetrabromidotantalum(V)], (TaBr 5 ) 2 , were obtained by recrystallization at 773 K. A first crystal structure study of (TaBr 5 ) 2 was reported by Rolsten [J. Am. Chem. Soc. (1958), 80, 2952-2953, who analysed the powder diffraction pattern and came to the conclusion that it crystallizes isotypically with (NbBr 5 ) 2 in a primitive orthorhombic cell. These findings are not in agreement with our current results of a monoclinic C-centred structure. (TaBr 5 ) 2 is isotypic with -(NbCl 5 ) 2 . The crystal structure contains [TaBr 6 ] octahedra sharing common edges forming [TaBr 5 ] 2 dimers. Two crystallographically independent dimers with symmetries m and 2/m and TaÁ Á ÁTa distances of 4.1574 (11) and 4.1551 (15) Å , respectively, are present in the structure.