The nature of the chemical bond in inorganic 6pi aromatic systems such as P4(2-), S4(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)2P4 x 2 NH3, bis(rubidium(18-crown-6))cyclotetraphosphide-cyclotetraarsenide-ammonia(1/3) (Rb@18-crown-6)2(P4)0.85(As4)0.15 x 3 NH3, both containing the 6pi aromatic cyclotetraphosphide anion, P4(2-), and the synthesis and crystal structure of bis(potassium(18-crown-6))cyclotetraarsenide (K@18-crown-6)2As4. As a common motive, all three compounds feature neutral molecules with a tripledecker-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, As4(2-), shows electron delocalization primarily through the lone pairs, as does P4(2-), and may consequently also be described as lone pair aromatic.

Compounds [Rb(18-crown-6)]4As14.6NH3 (1) and [Li(NH3)4]4P14.NH3 (2) were prepared by the reaction of Rb4As6 with SbPh3 and 18-crown-6 and by the reduction of white phosphorus with elemental lithium in liquid ammonia, respectively. Both were characterized by low-temperature single-crystal X-ray structure analysis. They were found to contain the Ci symmetrical Pn14(4-) anion (Pn = P, As), which consists of two nortricyclane-like Pn7-cages connected by a single bond. Molecular complexes of [Rb(18-crown-6)(NH3)]2[Rb(18-crown-6)]2As14 are formed in 1, which are connected to fanfold sheets via N-H...O bonds. The anion is isolated in 2, and N-H...N bonds result in the formation of {[Li(NH3)4](mu-NH3)2[Li(NH3)4]}2+ cationic complexes.

The syntheses and X-ray single-crystal low-temperature structures of the four new ammoniates [Li(NH 3 ) 4 ] 3 As 7´N H 3 (1), ] 3 As 7´8 NH 3 (2), Cs 3 As 7´6 NH 3 (3), and (Ph 4 P) 2 CsAs 7´5 NH 3 (4) are reported. The compounds were obtained by either direct reduction of As with Li/Cs in liquid NH 3 , solvation of Cs 4 As 6 /Rb 4 As 6 in liquid NH 3 , or by extraction of solid Cs 3 As 7 . While compound 1 contains isolated As 3À 7 polyanions, AsÀM contacts (M Na À Cs) lead to neutral 2À chains in 4, respectively.

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