Synthesis and Crystal Structure of Bis(tetraethylammonium) Octabromo‐triselenate(II)‐{dibromodiselenate(I)}, [NEt 4 ] 2 [Se 3 Br 8 (Se 2 Br 2 )], the Salt of a Mixed‐valence Bromoselenate(II, I) Anion

The brown crystals of [NEt 4 ] 2 [Se 3 Br 8 (Se 2 Br 2 )] ( 1 ) were obtained when selenium and bromine reacted in the solution of acetonitrile in the presence of tetraethylammonium bromide. The crystal structure of 1 has been determined by the X‐ray methods and refined to R = 0.0308 for 10433 reflections. The crystals are monoclinic, space group P2 1 with Z = 2 and a = 12.0393(3) Å, b = 11.8746(3) Å, c = 13.1946(3) Å, β = 96.561(1)° (123 K). In the solid state structure the anion of 1 is built up of Se 3 Br 8 unit which consists of a triangular arrangement of three planar SeBr 4 units sharing a common edge through two μ 3 ‐bridging Br atoms, and one Se 2 Br 2 molecule which is linked to one of μ 3 ‐bridging Br atoms. The three Se II atoms form a triangle which is almost perpendicular to the planes given by three SeBr 4 moieties. The contact between the μ 3 Br and the Se I atom of the Se 2 Br 2 molecule is 3.1711(8) Å and can be interpreted as a bond of the donor‐acceptor type with the μ 3 Br as donor and the Se 2 Br 2 molecule as acceptor. The terminal Se II ‐Br and μ 3 Br‐Se II bond lengths are in the ranges 2.3537(7)–2.4737(7) Å and 2.7628(7)–3.1701(7) Å, respectively. The bond lengths in coordinated Se 2 Br 2 molecule are: Se I ‐Se I = 2.2636(9) Å, Se I ‐Br = 2.3387(11) and 2.3936(8) Å.