A novel large polyoxometalate (NH₄)₂₁[{Na₂(H₂O)₃}Ni(H₂O)VOAs₄W₁₄₀]·53H₂O was synthesized by the reaction of Na₂7[NaAs₄W₄₀O140]·60H₂O with NiCl 2·6H₂O, NaVO₃·4H₂O and NH₄Cl in pH=4.5 aqueous solution.Its structure was determined by single crystal X-ray diffraction.Crystal data for the titled compound are: orthogonal system, Pmmn space group, a=2.3305(14) nm, b=2.9467(19) nm, c=1.2671(16) nm, V=8.701(9) nm³, Z=2, D_c=4.378 g/cm³, μ=27.372 mm^-1, R₁=0.0596, wR₂=0.1056 .The opposite two of four S2 sites of the big cyclic ligand [As₄W₄₀O140]^28- are disorderly occupied by one V⁵⁺ and one Ni²⁺ with the probability that each transition metal ion occupies the same S2 site being 50%, and the other two S2 sites are occupied by two Na⁺ which are bridged by a water molecule.IR spectra, TG and magnetic properties of the compound were also discussed.

Key words: polyoxometalate, crystal structure, tungsten, arsenic, vanadium