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Using 293 K diffractometer intensity data, the crystal structures of (NH 4 ) 2 SeBr 6  (1) and [H 3 N(CH 2 ) 3 NH 3 ]SeBr 6  (2) have been determined by single crystal X-ray technique and refined to a final R w  of 0.049 and 0.040, respectively. The dark red crystals of 1 are cubic (space group Fm3m) with a = 10.478(2) Å and Z = 4. This structure contains SeBe 6  2-  octahedra (point symmetry m3̄m, distance Se-Br: 2.577(2) Å in an antifluorite type arrangement of cations and anions. In contrast, the SeBr fi 2-ion in 2 is statically distorted (approximate point symmetry 3m, distance Se-Br min. 2,547(2), max. 2.595(2) Å. The dark red crystals are orthorhombic (space group Pnma) with a = 17.795(3), b = 7.5037(6), c = 10.476(1) Å and Z = 4. The results agree with the rules for the appearance of a static/dynamic stereochemical effect of the lone pair electrons given for TeX 6  2-  (X = Cl, Br, I) species (W. Abriel, Acta Crystallogr. B 42, 449 (1986)). Consequently these rules can be extended considering SeX 6  2-  as well. Basic structural data for all compounds containing AX 6  2-  ions (A = Se, Te; X = Cl, Br, I) known to date are given including the ⊿-values (R. D. Shannon, Acta Crystallogr. A 32, 751 (1976)) as a measure for the distortion of the octahedral anion.

zeitschrift für naturforschung b

On the Stereochemistry of the Lone-Pair Electrons in AX6E-Systems: Dynamically Distorted Anion in (NH 4 ) 2 SeBr 6 , Statically Distorted Anion in 1,3-Propanediammonium-Hexabromoselenate(IV) (C 3 N 2 H 12 )SeBr 6