A chain-shaped (H2O)4 cluster A is self-assembled in a metal phosphonate compound, [Al(phen)(AEDPH2)(AEDPH3)] · 4H2O (1) (AEDPH4 = 1-aminoethylidenediphosphonic acid, phen = 1,10-phenanthroline). This water cluster, and another chain-shaped (H2O)4 cluster B which exists in the previously reported compound [Zn(phen)(AEDPH2)2] · 4H2O (2), are investigated by X-ray crystallographic, thermal gravimetric analysis (TGA) and theoretical calculations. Both A and B collapse to ring-shaped (H2O)4 cluster C after geometry optimizations with 6-311+G(d, p) basis set at Hartree-Fock theory as well as density functional theory. Energy calculations reveal the stability is C > B > A, which is consistent with experiment results.