Synthesis and Crystal Structure of Tetrakis(tetramethylammonium) Bis[decabromotetraselenate(II)]-bis[dibromodiselenate(I)], [(CH3)4N]4[(Se4Br10)2(Se2Br2)2], the Salt of a Mixed-Valence Bromoselenate(II/I) Anion

Brown crystals of [NMe4](4)[(Se4Br10)(2)(Se2Br2)(2)] (1) were obtained from the reaction of selenium and bromine in acetonitrile in the presence of tetramethylammonium bromide The crystal structure of 1 was determined by X ray diffraction and refined to R = 0 0297 for 8401 reflections The crystals are monoclinic, space group P2(1)/c with Z = 4 and a = 12 646(3) angstrom, b = 16 499(3) angstrom, c = 16 844(3) angstrom, beta = 101 70(3)degrees (123 K) In the solid state structure the anion of 1 is built up of two [Se4Br10](2-) ions Each shows a triangular arrangement of three planar SeBr4 units sharing a common edge through two mu(3) bridging bromine atoms, and one SeBr2 molecule, which is linked to the Se-II atoms of two SeBr4 units, between the Se4Br102- ions a dimerized Se2Br2 molecule (Se4Br4) is situated and one Se-I atom of each Se2Br2 molecule has two weak contacts [3 3514(14) angstrom and 3 3952(11) angstrom] to two bromine atoms of one SeBr4 unit Four Se-1 atoms of a dimerized Se2Br2 molecule are in a almost regular planar tetraangular arrangement Contacts between the Se-II atom of the SeBr2 molecule and the Se-II atoms of two SeBr4 units are 3 035(1) angstrom and 3 115(1) angstrom, and can be interpreted as donor acceptor type bonds with the Se-II atoms of SeBr4 units as donors and the SeBr2 molecule as acceptor The terminal Se-II-Br and mu(3)-Br-Se-II bond lengths are in the ranges 2 3376(10) to 2 4384(8) angstrom and 2 8036(9) to 3 3183(13) angstrom respectively The bond lengths in the dimerized Se2Br2 molecule are Se-I-Se-I = 2 2945(8) angstrom and 31398(12), Se-I-Br = 2 3659(11) and 2 3689(10) angstrom