The structure of R3PBr2 compounds in the solid state and in solution;: geometrical dependence on R, the crystal structures of tetrahedral ionic Et3PBr2 and molecular trigonal bipyramidal (C6F5)3PBr2

Twenty-one triorganophosphorus dibromide compounds, R3PBr2 (R-3 = substituted aryl, mixed aryl-alkyl, triaryl or trialkyl) have been synthesized from diethyl ether solution and characterised by analytical and P-31-{H} NMR data in CDCl3 solution, the vast majority being reported for the first time. All but one of the compounds are ionic, [R3PBr]Br in CDCl3 solution, in keeping with analogous species containing iodine or chlorine, [R3PX]X (X = I or Cl) according to P-31-{H} NMR studies. In contrast, (C6F5)(3)PBr2 has a molecular five-co-ordinate trigonal bipyramidal structure both in CDCl3 solution and in the solid state. The single crystal structure of this compound has been determined and it represents the only reported R3PBr2 species which contains a five-co-ordinate phosphorus atom. It has D-3 symmetry and perfect trigonal bipyramidal geometry. Why (C6F5)(3)PBr2 is the only R3PBr2 compound which adopts trigonal bipyramidal geometry is reasoned to be due to the very low basicity of the parent tertiary phosphine, which favours this geometry for the dihalogen adducts, a phenomenon previously observed for dichlorine adducts of tertiary phosphines and dihalogen adducts of tertiary arsines. The crystal structure of the first non-solvated trialky-phosphine dibromide compound, Et3PBr2, has also been determined and contains a tertrahedral phosphorus atom, exhibiting a long Br ... Pr contact, 3.303(2) Angstrom, and is probably better described as ionic, [Et3PBr]Br, with significant cation-anion interaction.