Sandwich-type polyoxometalates of the later lanthanide ions. Syntheses and structures of [(A-XW₉O₃₄)₂(H₂OM)₃CO₃]¹¹⁻ (X = P⁵⁺, As⁵⁺) (M = Tb³⁺, Dy³⁺, Er³⁺)

Six new complexes were synthesized in a 0.1 M aqueous carbonate solution and characterized by elemental analyses, thermogravimetric analyses, infrared spectroscopy and X-ray single crystal structure analyses. The single-crystal structure analysis was on K₁₁[(A-AsW₉O₃₄)₂(H₂ODy)₃CO₃]·22H₂O. The results revealed that the complexes are isomorphic and their structures consist of two lacunary [A-XW₉O₃₄]⁹⁻ Keggin moieties linked by a (OMOH₂)₃C belt, forming an A-type sandwich structure. The is tridentate and fixes three M³⁺ centers together by its oxygens. Each M³⁺ cation is mono-capped trigonal-prismatic coordination, achieved by two terminal oxygens of an edge-shared pair of WO₆ octahedra to each [A-XW₉O₃₄]⁹⁻ and two oxygens from the belt, and the cap by one water.