The incongruent solvation of MI4P6 species (MI = K, Rb, Cs) in liquid ammonia leads to a broad variety of polyphosphides such as P73−, P113−, and the putatively aromatic P42− and P5−, which we investigated by using NMR spectroscopy and single-crystal X-ray structure analysis. The structures of Cs2P4⋅2 NH3, (K@crown-6)3K3(P7)2⋅10 NH3, Rb3P7⋅7 NH3, and (Rb@crown-6)3P7⋅6 NH3 are discussed and compared. The electron localization function ELF is used in a comparison of the chemical bonding of various phosphorus species. The variances of the basin populations provide a well-established measure for electron delocalization and therefore aromaticity. While comparable variance is calculated for P42− and P5− it is observed in the lone pairs rather than in the basin populations of the bonds as in the prototypical aromatic hydrocarbons such as benzene or the cyclopentadienide anion. For this behavior, the term “lone pair aromaticity” is proposed.
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