Compound Na[Cu(en)2]2[HW7O24]·5H2O (1, en = ethanediamine), as the first example of a heptatungstate polymer, has been synthesized in H2O/dmso and characterized by single-crystal X-ray diffraction, IR, UV/Vis, and ESI-MS. The [HW7O24]5– cluster in 1, as an inorganic hexadentate ligand, coordinates to two Na+ and four [Cu(en)2]2+ cations to construct a 3D extended structure. The solid-state stability of 1 at room temperature was proven by X-ray powder diffraction and digital camera techniques. The electrospray mass spectra suggest that most of the [HW7O24]5– dissociates into relatively more stable [W6O19]2– species in aqueous solution because the coordination bonds around the [HW7O24]5– anions are broken by the solvation effect of water on the the copper cation. These results demonstrate that a transition-metal-complex cation plays an important role in the isolation and stabilization of a highly charged polyoxometalate anion.