Aromaticity of regular heptagonal trianion in the (M = Li, Na, K, Rb, and Cs) species

Geometries, electronic structures and vibrational frequencies of alkali metal– P 7 3 - compounds MP 7 2 - (M = Li, Na, K, Rb, and Cs) were examined using density functional theory (DFT) methods. To our knowledge, these compounds are first reported here. Calculation results show that regular heptagonal P 7 3 - trianion can coordinate with alkali metal atoms to form the pyramidal structures MP 7 2 - species that maintain the planar cyclo - P 7 3 - trianion structure. On the basis of the molecular orbital (MO) analysis and nucleus-independent chemical shifts (NICS) analysis, we revealed that regular heptagonal P 7 3 - trianion exhibits characteristic of π aromaticity with 10 delocalized π electrons and maintains its structural and electronic integrity inside the pyramidal MP 7 2 - (M = Li, Na, K, Rb, and Cs) clusters.