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The reactions of Te, TeCl 4  and MCl 5  (M = Nb, Ta) in a sealed evacuated ampoule at 170 °C yield Te 4 (NbCl 6 ) 2  and Te 4 (TaCl 6 ) 2  as black, moisture sensitive crystals. Te 4 (TaBr 6 ) 2  is obtained from Te 2 Br and TaBr 3  at 175 °C, and Te 4 (Ta 2 OBr 10 ) is obtained in small yields as byproduct. The crystal structure determinations (all triclinic, P1, lattice constants for Te 4 (NbCl 6 ) 2 : a = 654,2 pm, b = 673,9 pm, c = 1200,3 pm, α = 94,54°, β = 97,59°, γ = 85,29°, V = 521,5 -10 6  pm 3 , Z = 1; for Te 4 (TaCl 6 ) 2 : a = 651,2(5) pm, b = 747,2(7) pm, c = 1107(1) pm, a = 100,33(8)°, β = 102,29(8)°, y = 89,24(8)°, V = 517,4 -10 6  pm 3 , Z = 1; for Te 4 (TaBr 6 ) 2 : a = 680,9(3) pm. b = 710,4(3) pm, c = 1239,9(5) pm, α = 95,29(4)°, β = 98,05(4)°, γ = 85,31(4)°, V = 589,8 -10 6  pm 3 , Z = 1; for Te 4 (Ta 2 OBr 10 ): a = 692,2(5) pm, b = 877,2(7) pm. c = 917(1) pm, α = 81,91(9)°, β = 86,39(9)°, γ = 74.23(6)°, V= 530,3-10 6  pm 3 , Z = 1) have shown, that all compounds contain nearly square planar Te 4  2+  ions. Te 4 (TaCl 6 ) 2  and Te 4 (TaBr 6 ) 2  have distorted octahedral TaCl 6-  and TaBr 6-  ions. Te 4 (NbCl 6 ) 2  is isostructural to Te 4 (TaBr 6 ) 2  as was shown by the coincident lattice constants. Te 4 (Ta 2 OBr 10 ) is isomorphous to Te 4 (Nb 2 OCl 10 ) and contains a Ta 2 OBr 10  2-  ion in the form of two edge-sharing octahedra with the O atom in the centre of a linear Ta=O=Ta bridge. All structures adopt simple structure types. The arrangement of the ions in the compounds Te 4 (MX 6 ) 2  (M = Nb, Ta, W; X = Cl. Br) corresponds to the CdI 2  structure type, and in Te 4 (Ta 2 OBr 10 ) to the CsCl structure type. An analysis of the Te···Te bond length in Te 4  2+  and the distance of Te···X (X = Cl, Br) contacts to the nearest neighbour halide anions in several Te 4  2+ -halometallates shows a clear correlation. Shortening of the Te···X contacts elongates the Te-Te bonds by a donor interaction of the halide ions with Te-Te-σ* orbitals.

zeitschrift für naturforschung b

Das Te 4  2+ -Polykation in den Strukturen von Te 4 (NbCl 6 ) 2 , Te 4 (TaCl 6 ) 2 , Te 4 (TaBr 6 ) 2  und Te 4 (Ta 2 OBr 10 ) / The Te 4  2+  Polycation in the Structures of Te 4 (NbCl 6 ) 2 , Te 4 (TaCl 6 ) 2 , Te 4 (TaBr 6 ) 2  and Te 4 (Ta 2 OBr 10 )