Nature of the Chemical Bond in Polypnictides:  The Lone Pair Aromatic Anions P₄^2- and As₄^2-

Abstract

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.

Chemical Bond in the Cyclic Anions P64− and As64−: Synthesis, Crystal Structure, and Electron Localization Function of {Rb[18]crown‐6)}2Rb2As6⋅6 NH3

“…This result is surprising since the P 6 4À ring was once thought to be a 10p aromatic ground state system in Hückel and Hartree Fock (HF) calculations. [2,24,25] These results are not verified by ab initio calculations at the HF [15,26] or B3LYP [15] level of theory which include the evaluation of the Hesse matrix. The topological analysis of the ELF [17][18][19][20][21][22][23] of the P 6 4À ion indicates a localized P=P double bond (Figure 2 d,e).…”

“…Thus, the arsenicarsenic bonds in the {Rb 4 As 6 } ion complex are better described as depopulated single bonds. In contrast to As 4 2À , [15] no lone pair aromaticity seems to be present in As 6 4À , which also means that this anion should not be 10p aromatic.…”

“…Upon solvation in liquid ammonia, these hexaarsenides form heptaarsenide As 7 3À , [13] tetradecaarsenide [(As 7 ) 2 ] 4À , [14] and the lone pair aromatic cyclotetraarsenide As 4 2À as crystalline products. [15,16] However, in contrast to the hexaphosphide anion, the hexaarsenide anion seems to be moderately stable in liquid ammonia solutions, and we have now been able to dissolve Rb 4 As 6 congruently in it. We isolated the As 6 4À ion as an ion complex in which only four rubidium cations coordinate to it instead of fourteen as in the solid-state compound Rb 4 As 6 .…”

Reactions of Sn(NMe2)2 with MPHCy: The Effects of Alkali Metal Phosphide Coupling (Cy=Cyclohexyl; M=Li, Na, K, Rb)

“…Both values agree well with previous findings in P species, like P 6 H 8 , P 4 , P 7 3À , P 11 3À , P 5 À and P 4 2À . [32][33][34] The covariance matrix contains information concerning the delocalisation of electrons between neighbouring basins. The covariance of the P core basins C(P) shows that the protonated disynaptic valence basin VA C H T U N G T R E N N U N G (P, H) (the PÀH bond, orange in Figure 8), the monosynaptic valence basin V(P) (the P lone pair, red) and the disynaptic valence basin VA C H T U N G T R E N N U N G (P, Sn) (P À Sn bond, yellow) are involved in this delocalisation.…”

Stepwise Synthesis and Coordination Compound of an Inorganic Cryptand

“…[1] They are currently the most widely used ligands in the preparation of crystalline materials containing Zintl anions. [2] Formal substitution of the C 2 H 4 groups in crown ethers by SiR 2 groups yields cyclosiloxanes. These inorganic crown ethers have been shown recently to act also as ligands.…”