A novel heteropolyoxometalate (NH_4)_(19)Na_2 [ {WO (H_2O) }_3 (AsW_8O_(31) )_3 ]· 45H_2O was synthesized by the reaction of Na_2WO_4 · 2H_2O with NaAsO_2· 6H_2O and NH4CI in pH = 3.0 aqueous solution. The single crystal X-ray diffraction study shows that the crystal belongs to monoclinic system, space group C2/c, with a =3.1165(11) nm, 6 = 1.7388(7) nm, c = 2.2789(9) nm, β = 97.342(6)°, V= 12.248(8) nm3, Z = 4, R_1 = 0.0571, wR_2 = 0.0741 [I>2σ(I)]. The polyanion [ {WO (H_2O)}_3(AsW_8O_(31)]~(21-) consists of three new {AsW_8O_(31)} subunits related through a C3 axis and joined up by three extra { WO (H_2O)} group. Each {WO(H_2O) } joints two AsW_8O_(31) subunits by sharing two cis oxygen atoms with each other, the polyanion [ { WO (H_2O) }_3 (AsW_8O_(31) )_3]~(21-) has approximately D_(3h) symmetry. The compound was also characterized by ~(183)W NMR, IR and TG.

Key words: HETEROPOLYACID, TUNGSTATE, TUNGSTEN, ARSENIC, CRYSTAL STRUCTURE, XRD, NMR, IR, TG