Three new sandwich-type polyoxometallates [(A-a-AsW9O34)2(MOH2)3(CO3)]11 M=Y(III) 1, Yb(III) 2 and Sm(III) 3 have been synthesized by the reaction of the trilacunary [A-a-AsW9O34]9 ligand with M(III) in a carbonate solution, and characterized by elemental analysis, IR, 13C NMR, UV–Vis spectroscopy, TGA, DSC and single crystal structure analysis. The crystal structure of the complexes consist of two lacunary A-a-[AsW9O34]9 Keggin moieties which are linked by a M3CO3 belt into an assembly of virtual C3v symmetry. The CO3 2 anion acts as a tridentate ligand and fixes three M(III) centres together by its oxygen atoms. The syntheses of the complexes require the presence of a carbonate anion and their decomposition take place with the loss of it. EachM(III) cation adopts a mono-capped trigonal-prismatic coordination, the local 3-fold axis of the MO6 group lying in the M3CO3 plane. The trigonal prismatic geometry is achieved by two the terminal oxygen atoms of an edge shared paired of WO6 octahedra to each A-a-[AsW9O34]9 moiety and two oxygen atoms from the belt, and the cap by one external water ligand.
