Nature of the chemical bond in polypnictides:: The lone pair aromatic anions P42- and As42-

The nature of the chemical bond in inorganic 6 pi aromatic systems such as P-4(2-), S-4(2+), or S2N2 is a matter of particular interest because the phenomenon of aromaticity is not as well established in these compounds as it is in the classic aromatic hydrocarbons. Here we present the synthesis, NMR spectra, and crystal structures of bis(potassium(18-crown-6))cyclotetraphosphide-ammonia(1/2) (K@18-crown-6)(2)P-4 center dot 2NH(3), bis(rubidium(18-crown-6))cyclotetraphosphide-cyclotetraarsenide-ammonia(1/3) (Rb@18-crown-6)(2)(P-4)(0.85)(AS(4))(0.15)center dot 3NH(3), both containing the 6 pi aromatic cyclotetraphosphicle anion, P-4(2-), and the synthesis and crystal structure of bis(potassium(18-crown6))cyclotetraarsenide (K@18-crown-6)(2)As-4. As a common motive, all three compounds feature neutral molecules with a tripleclecker-like coordination of the cyclotetrapnictide anion between two crown ether-coordinated alkali metal cations. With ab initio calculations on the HF level and by employing the concept of the electron localization function ELF, we established that the cyclotetraarsenide anion, AS(4)(2-), shows electron delocalization primarily through the lone pairs, as does P-4(2-), and may consequently also be described as lone pair aromatic.