The Te-4(2+) polycation in the structures of Te-4(NbCl6)(2), Te-4(TaCl6)(2), Te-4(TaBr6)(2) and Te-4(Ta2OBr10)

The reactions of Te, TeCl4 and MCl(5) (M = Nb, Ta) in a sealed evacuated ampoule at 170 degrees C yield Te-4(NbCl6)(2) and Te-4(TaCl6)(2) as black, moisture sensitive crystals. Te-4(TaBr6)(2) is obtained from Te2Br and TaBr5 at 175 degrees C, and Te-4(Ta2OBr10) is obtained in small yields as byproduct. The crystal structure determinations (all triclinic, <P(1)over bar>, lattice constants for Te-4(NbCl6)(2): a = 654,2 pm, b = 673,9 pm, c = 1200,3 pm, alpha = 94,54 degrees, beta = 97,59 degrees, gamma = 85,29 degrees, V = 521,5 . 10(6) pm(3), Z = 1; for Te-4(TaCl6)(2): a = 651,2(5) pm, b = 747,2(7) pm, c = 1107(1) pm, alpha = 100,33(8)degrees, beta = 102,29(8)degrees; gamma = 89,24(8)degrees, V = 517,4 . 10(6) pm(3), Z = 1; for Te-4(TaBr6)(2): a = 680,9(3) pm, b = 710,4(3) pm, c = 1239,9(5) pm, alpha = 95,29(4)degrees, beta = 98,05(4)degrees, gamma = 85,31(4)degrees, V = 589,5 . 10(6) pm(3), Z = 1; for Te-4(Ta2OBr10): a = 692,2(5) pm, b = 877,2(7) pm, c = 917(1) pm, alpha = 81,91(9)degrees, beta = 86,39(9)degrees, gamma = 74,23(6)degrees, V = 530,3 . 10(6) pm(3), Z = 1) have shown, that all compounds contain nearly square planar Te-4(2+) ions. Te-4(TaCl6)(2) and Te-4(TaBr6)(2) have distorted octahedral TaCl6- and TaBr6- ions. Te-4(NbCl6)(2) is isostructural to Te-4(TaBr6)(2) as was shown by the coincident lattice constants. Te-4(Ta2OBr10) is isomorphous to Te-4(Nb2OCl10) and contains a Ta2OBr102- 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 CdI2 structure type, and in Te-4(Ta2OBr10) 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-sigma* orbitals.