Density functional theory (DFT) method has been used to investigate geometries, electronic structures, and vibrational frequencies of alkali metal P4M2 (M = Li, Na, and K) clusters and alkali earth metal P4M (M = Be, Mg, and Ca) clusters. Calculation results show that the square p(4)(2-) dianion can coordinate with metal atoms to form the pyramidal P4M and the dipyramidal P4M2 complexes maintaining the square p(4)(2-) dianion structure. Molecular orbital (MO) analysis revealed that the square planar p(4)(2-) dianion exhibits characteristic of pi-aromaticity with six delocalized pi electrons and maintains its structural and electronic integrity inside the P4M2 and P4M clusters. (C) 2004 Elsevier B.V. All rights reserved.