STAGES OF HALOGENATION AND DISSOCIATION O F A DIPHOSPHONIO-1,2,4-TRIPHOSPHOLIDE

Reductive condensation of Ph(3)P=C(SiMe(3))(2) and PCl3 or PBr3 yields the ionic halides of the 3,5-bis(triphenylphosphonio)-1,2,4-triphosph olide cation 5. The compounds can take up two moles of halogen X(2). I n the first step henylphosphoranediyl)trihalo-1,2,4-triphospholanes 6 are formed. Low temperature P-31 NMR spectroscopy shows them to exist in two conformeric forms. In polar solvents heterolytic dissociation o f the phosphorus-halogen bond in the 4-position occurs, The molecular structure of 5 demonstrates a cyclic delocalized bonding system. Chlor ination or bromination (to give 6) effects little structural change wi th exception of a large increase of the PP bund length. The phosphorus -halogen bonds in 6 are very long and rather different in length. The overall situation as well as tile structural details can be rationaliz ed to result ti-om a negative hyperconjugation between the ylidic carb on atoms and the phosphorus-halogen bonds.