Synthesis and Crystal Structure of Bis(tetraphenylphosphonium) Hexabromo‐diselenate(II)‐bis{dibromodiselenate(I)}, [PPh 4 ] 2 [Se 2 Br 6 (Se 2 Br 2 ) 2 ], the Salt of a Mixed‐valence Bromoselenate(II,I) Anion

The Red crystals of [PPh 4 ] 2 [Se 2 Br 6 (Se 2 Br 2 ) 2 ] ( 1 ) were obtained when selenium and bromine reacted in the solution of acetonitrile in the presence of tetraphenylphosphonium bromide. The crystal structure of 1 has been determined by X‐ray diffraction and refined to R = 0.0201 for 4024 reflections. The crystals are triclinic, space group $\rm P{\bar 1}$ with Z = 2 and a = 11.2757(4) Å, b = 12.3347(5) Å, c = 12.4948(5) Å, α = 113.152(4)°, β = 114.745(4)°, γ = 91.208(3)° (120(2) K). In the solid state the anion of 1 is built up of the Se 2 Br 6 core and two Se 2 Br 2 molecules each of which is linked to one of the trans ‐positioned terminal Br t atoms of the Se 2 Br 6 core. The central Se 2 Br 6 part consists of a nearly planar arrangement of two planar SeBr 4 units sharing a common edge through two μ 2 ‐bridging Br atoms. The contact between the Br t and the Se I atom of the Se 2 Br 2 molecule is 3.0872(5) Å and can be interpreted as a bond of the donor‐acceptor type with the Br t as donor and the Se 2 Br 2 molecule as acceptor. The terminal Se II –Br and μ 2 Br–Se II bond lengths are 2.3654(4), 2.6699(5) Å and 2.5482(5), 3.0265(5) Å, respectively. The bond lengths in the coordinated Se 2 Br 2 molecule are: Se I –Se I = 2.2686(5) Å, Se I –Br = 2.3779(5) and 2.3810(5) Å.