Das Te42+-Polykation in den Strukturen von Te4(NbCl6)2, Te4(TaCl6)2, Te4(TaBr6)2 und Te4(Ta2OBr10)

The reactions of Te, TeCl4 and MC15 (M = Nb, Ta) in a sealed evacuated ampoule at 170 °C yield Te4(NbCl6)2 and Te4(TaCl6j2 as black, moisture sensitive crystals. Te4(TaBr6)2 is obtained from Te2Br and TaBr3 at 175 °C, and Te4(Ta2OBr1()) is obtained in small yields as byproduct. The crystal structure determinations (all triclinic, PI, lattice constants for Te4(NbCl6)2: a = 654,2 pm, b = 673,9 pm, c = 1200,3 pm, a = 94,54°, ß = 97,59°, y = 85,29°, V = 521,5 -106 pm3, Z = 1; for Te4(TaCl6)2: a = 651,2(5) pm, b = 747,2(7) pm, c = 1107(1) pm, a = 100,33(8)°, ß = 102,29(8)°, y = 89,24(8)°, V = 517,4-106 pm3, Z = 1; for Te4(TaBr6)2: a = 680,9(3) pm. b = 710,4(3) pm, c = 1239,9(5) pm, a = 95,29(4)°, ß = 98,05(4)°, y = 85,31(4)°, V = 589,8-106 pm3, Z = 1; for Te4(Ta2OBr10): a = 692,2(5) pm, b = 877,2(7) pm. c = 917(1) pm, a = 81,91(9)°, ß = 86,39(9)°, y = 74.23(6)°, V^= 530,3-106 pm3, Z = 1) have shown, that all compounds contain nearly square planar Te42+ ions. Te4(TaCl6)2 and Te4(TaBr6)2 have distorted octahedral TaCl6_ and TaBr6_ ions. Te4(NbCl6)2 is isostructural to Te4(TaBr6)2 as was shown by the coincident lattice constants. Te4(Ta2OBr10) is isomorphous to Te4(Nb2O Cl10) and contains a Ta2O Br102~ ion in the form of two edge-sharing octahedra with the O atom in the centre of a linear Ta=0=Ta bridge. All structures adopt simple structure types. The arrangement of the ions in the compounds Te4(MX6)2 (M = Nb, Ta, W; X = Cl. Br) corresponds to the Cdl2 structure type, and in Te4(Ta2OBr1()) to the CsCl struc­ ture type. An analysis of the T e-T e bond length in Te42+ and the distance of Te -X (X = Cl, Br) contacts to the nearest neighbour halide anions in several Te42+-halometallates shows a clear correlation. Shortening of the Te---X contacts elongates the T e-T e bonds by a donor interaction of the halide ions with T e-T e-a* orbitals.