The synthesis and molecular structure of a monomeric diplatinum(II) complex composed of mono-lacunary α-Keggin polyoxometalate is described. The polyoxometalate, Cs(3)[α-PW(11)O(39){cis-Pt(NH(3))(2)}(2)]·8H(2)O (Cs-1), afforded by a stoichiometric reaction of mono-lacunary Keggin polyoxotungstate with cis-diamminedichloroplatinum(ii) in water, followed by crystallization from water, was obtained as analytically pure, homogeneous, yellow crystals. The compound Cs-1 was characterized by elemental analysis, thermogravimetric/differential thermal analysis (TG/DTA), Fourier transform infrared (FTIR) and UV-visible spectroscopy, solution (1)H and (31)P nuclear magnetic resonance (NMR), and X-ray crystallography. The single-crystal X-ray structure analysis revealed that the two cis-platinum(ii) moieties, [cis-Pt(NH(3))(2)](2+), were coordinated each to two oxygen atoms in a mono-vacant site of [α-PW(11)O(39)](7-) with asymmetric configuration, resulting in an overall C(1) symmetry. Furthermore, hydrogen evolution from an EDTA·2Na (ethylenediamine tetraacetic acid disodium salt) aqueous solution under visible-light irradiation (≥400 nm) was achieved by using polyoxoanion 1 and titanium dioxide.