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)

Abstract

The bromoarsenates(lll) [Et4N]3[As3Br12] (1). [Ph4P]2[As2Br8] (2), [(n-prop)4N]2[As2Br8] (3) and {[pipH]3[As2Br9] · [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 [As3Br12] displays an effectively regular octahedral coordination in one of the two independent anions in the unit cell of 1. The As-Br distances are 2.671 (1) - 2.672(2) Å 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 As - Br terminal bonds (2.404(2)-2.409(2) Å). The [As2Br8]2- anions in 2 and 3 are centrosymmetric with the As atoms displaying a square-pyramidal coordination. A structural correlation of opposite As-Br distances in the linear three-centre Br-As···Br interactions is presented. The sum of the bond valences in bromoarsenates(III) is a minimum for the regular octahedral geometry, reflecting, thereby, the influence of the As 4s-orbital in the antibonding 2a1g MO. An empirical expression for As-Br distances in bromoarsenates(III) is derived.