Three Strandberg-type polyoxometalate compounds [Cu(L)2(H2O)2]2H2[P2Mo5O23]·2CH3OH (1), [Cu(L)2(H2O)]H2[Cu(L)2(P2Mo5O23)]·4H2O (2), [Cd(L)2(H2O)2]2H2[P2Mo5O23]·2CH3OH (3), (L=pyridine-2-carboxamide) have been synthesized and structurally characterized by elemental analysis, spectroscopic methods (IR and UV–vis) as well as single crystal X-ray diffraction. Single-crystal X-ray structural analyses indicate that 1 and 3 are isostructural and crystallized in monoclinic, space group I2/a. Biological studies have indicated that compounds 1–3 exhibit broad and effective activities against the tested cells. A synergistic effect involving L, metal and P2Mo5 could probably explain the improved growth-inhibiting properties. Both coordination mode and the type of metal ion play significant roles in these compounds cytotoxicity.