A novel tetrahedral mu-AsO4-bridging hexadecanuclear Ni-substituted silicotungstate (ST) Na21H10[(AsO4)-{Ni-8(OH)(6)(H2O)(2)(CO3)(2)(A-alpha-SiW9O34)(2)}(2)]center dot 60H(2)O (1) was made by the reactions of trivacant [A-alpha-SiW9O34](10)-({SiW9}) units with Ni2+ cations and NaAsO4 center dot 12H(2)O and characterized by IR spectrometry, elemental analysis, thermogravimetric analysis (TGA), and powder X-ray diffraction (PXRD). 1 contains a novel polyoxoanion [(AsO4){Ni-8(OH)(6)(H2O)(2)(CO3) (2)(A-alpha-SiW9O34)(2)}(2)](31-) built by four trivacant Keggin [A-alpha-SiW9O34](10-) fragments linked through an unprecedented [(AsO4){Ni-8(OH)(6)(H2O)(2)(CO3)(2)}(2)](9+) cluster, where the tetrahedral AsO4 acts as an exclusively mu(2)-bridging unit to link multiple Ni centers; such a connection mode appears for the first time in polyoxometalate chemistry. Furthermore, the electrochemical and catalytic oxidation properties of compound 1 have been investigated.