Synthesis and Crystal Structure of >Bis(tetrapropylammonium) Hexabromotellurate(IV)-bis{dibromoselenate(II)}, [NnPr4]2[TeBr6(SeBr2)2], the Salt of a Mixed-valence Bromotellurate(IV)-Selenate(II) Anion

Dark brown crystals of [NPr4]2[TeBr6(SeBr2)2] (1) were obtained when selenium and bromine (1:1) were allowed to react in acetonitrile solution in the presence of tellurium(IV) bromide and tetrapropylammonium bromide. The salt 1 crystallizes in the monoclinic space group P21/n with the cell dimensions a = 14.7870(3)A, b = 9.5523(3)A, c = 16.7325(3)A, beta = 110.56(10)deg (at 123(2) K). In the solid state the [TeBr6(SeBr2)2]2- anion contains a nearly regular [TeBr6] octahedron in which the four equatorial bromo ligands have developed bonds to Se(II) atoms of the SeBr2 molecules. The contacts between the bridging bromo and the Se(II) atoms of the SeBr2 molecules are 3.0000(4) and 3.0561(4)A, and can be interpreted as bonds of the donor-acceptor type with the bridging bromo ligands as donors and the SeBr2 molecules as acceptors. The Te(IV)-Br distances are in the range 2.6816(3)-2.7131(3)A and the Se(II)-Br bond lengths in the coordinated SeBr2 molecules are 2.3548(4) and 2.3725(4)A.

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