A Remarkable Member of the Polyoxometalates: The Eight-Nickel-Capped α-Keggin Polyoxoazonickelate

The eight-nickel-capped polyoxoazonickelate, [Ni20(OH)24(MMT)12(SO4)](NO3)2·6H2O (1; MMT = 2-mercapto-5-methyl-1,3,4-thiadiazole), has been synthesized, which has an α-Keggin structure with eight nickel caps. In this structure, the polyatom is the late transition metal NiII; the central heteroatom is S, and the organic terminal ligand becomes the primary part of the Keggin structure. This is a Keplerate-type cluster, which shows a central NiII12 cuboctahedron formed by the 12 NiII centers of the classical α-Keggin core and a NiII8 hexahedron formed by the eight nickel caps.