<i>Cyclo</i> ‐P ₅ ⁻ Revisited: The Surprisingly Stable Uncoordinated Pentaphospholide Anion

Abstract

Abstract The addition of [2.2.2]cryptand to alkali metal heptaphosphides M 3 P 7 (M = Na, K) leads to the formation of [M([2.2.2]cryptand)][ cyclo ‐P 5 ] salts. Although the cyclo ‐P 5 − anion is spectroscopically known since the 1980s, it was so far prepared and handled only in solution. By complexing the alkali metal cation with [2.2.2]cryptand, the product was found to be surprisingly stable in the solid state. Cyclo ‐P 5 − , which is isolobal to the well‐known cyclopentadienide anion, was now characterized crystallographically for the first time in its uncoordinated form and in the presence of the weakly coordinating cations [M(2.2.2‐cryptand)] + (M = Na, K). The structural elucidation proves its planar D 5   h symmetry, not only as ligand in different sandwich complexes but also in its uncoordinated form. C yclo ‐P 5 − was further characterized by UV/Visis spectroscopy in solution and in the solid state by Raman and 31 P MAS NMR spectroscopy. The reaction of [Na([2.2.2]cryptand)][ cyclo ‐P 5 ] with LiCp* and FeCl 2 yields the ferrocene derivative [Cp*Fe( cyclo ‐P 5 )].