Geometries, electronic structures and vibrational frequencies of alkali metal-p(7)(3-) compounds MP72- (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 MP72- species that maintain the planar 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 pi aromaticity with 10 delocalized pi electrons and maintains its structural and electronic integrity inside the pyramidal MP72- (M = Li, Na, K, Rb, and Cs) clusters. (c) 2005 Elsevier B.V. All rights reserved.