The addition of [2.2.2]cryptand to alkali metal heptaphosphides M3P7 (M = Na, K) leads to the formation of the salts [M([2.2.2]cryptand)][cyclo-P5]. 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. The structural elucidation proves its planar D5h symmetry, not only as ligand in different sandwich complexes but also in its uncoordinated form. Cyclo-P5− was further characterized by UV-vis spectroscopy in solution and in the solid-state by Raman and 31P MAS NMR spectroscopy. Accompanying DFT studies were performed to support the experimental results.
