Synthesis and Crystal Structure of Tetrakis(tetramethylammonium) Bis[decabromotetraselenate(II)]‐bis[dibromodiselenate(I)], [(CH 3 ) 4 N] 4 [(Se 4 Br 10 ) 2 (Se 2 Br 2 ) 2 ], the Salt of a Mixed‐Valence Bromoselenate(II/I) Anion

Brown crystals of [NMe 4 ] 4 [(Se 4 Br 10 ) 2 (Se 2 Br 2 ) 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 P 2 1 / c with Z = 4 and a = 12.646(3) Å, b = 16.499(3) Å, c = 16.844(3) Å, β = 101.70(3)° (123 K). In the solid‐state structure, the anion of 1 is built up of two [Se 4 Br 10 ] 2– ions. Each shows a triangular arrangement of three planar SeBr 4 units sharing a common edge through two μ 3 ‐bridging bromine atoms, and one SeBr 2 molecule, which is linked to the Se II atoms of two SeBr 4 units; between the Se 4 Br 10 2– ions a dimerized Se 2 Br 2 molecule (Se 4 Br 4 ) is situated and one Se I atom of each Se 2 Br 2 molecule has two weak contacts [3.3514(14) Å and 3.3952(11) Å] to two bromine atoms of one SeBr 4 unit. Four Se I atoms of a dimerized Se 2 Br 2 molecule are in a almost regular planar tetraangular arrangement. Contacts between the Se II atom of the SeBr 2 molecule and the Se II atoms of two SeBr 4 units are 3.035(1) Å and 3.115(1) Å, and can be interpreted as donor‐acceptor type bonds with the Se II atoms of SeBr 4 units as donors and the SeBr 2 molecule as acceptor. The terminal Se II –Br and μ 3 ‐Br–Se II bond lengths are in the ranges 2.3376(10) to 2.4384(8) Å and 2.8036(9) to 3.3183(13) Å, respectively. The bond lengths in the dimerized Se 2 Br 2 molecule are: Se I –Se I = 2.2945(8) Å and 3.1398(12), Se I –Br = 2.3659(11) and 2.3689(10) Å.