Cyclo-P₅ ^- Revisited: The Surprisingly Stable Uncoordinated Pentaphospholide Anion

The addition of [2.2.2]cryptand to alkali metal heptaphosphides M₃P₇ (M = Na, K) leads to the formation of [M([2.2.2]cryptand)][cyclo-P₅] salts. Although the cyclo-P₅ ^- 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-P₅ ^-, 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. Cyclo-P₅ ^- was further characterized by UV/Visis spectroscopy in solution and in the solid state by Raman and ³¹P MAS NMR spectroscopy. The reaction of [Na([2.2.2]cryptand)][cyclo-P₅] with LiCp* and FeCl₂ yields the ferrocene derivative [Cp*Fe(cyclo-P₅)].