Regular Octahedral Coordination of As(III) in the Anion [As3Br12]3_ Structural Correlation of the Antibonding Influence of the As 4s-Orbital in Bromoarsenates(III)

The brom oarsenates(lll) [Et4N]3[A s3B rl2] (1). [Ph4P]2[A s2Brs] (2), [(/z-prop)4N]2[A s2Brs] (3) and {[pipH]3[A s2Bn,] • [pipH]Br} (4) have been prepared and their structures established by X-ray structural analysis. The central As atom in the novel face-bridged trioctahedral species [A s;Br,2] displays an effectively regular octahedral coordination in one of the two independent anions in the unit cell of 1. The A s—Br distances are 2.671 (1) — 2.672(2) Ä with Br—A s—Br angles in the range 88.8(1)—91.2(1)°. In contrast the outer As atoms exhibit a severely distorted octahedral geometry with three short A s -B r terminal bonds (2 .404(2)—2.409(2) Ä ). The [A s:Br,s]2 anions in 2 and 3 are centrosymmetric with the As atoms displaying a square-pyramidal coordination. A structural correlation of opposite A s—Br distances in the linear three-centre B r—As Br interactions is presented. The sum of the bond valences in bromoarsenates(III) is a minimum for the regular octahedral geom etry, reflecting, thereby, the influence of the As 4s-orbital in the antibonding 2 a lg MO. An empirical expression for As —Br distances in bromoarsenates(III) is derived.