Polyoxometalates often display a unique multitude of properties based on flexibility of their sizes, shapes, charge densities, and reversible redox potentials. Incorporation of multiple of rare earth metal ions in lacunary polyoxoanion fragments can lead to species with various stoichiometries and structural features combined with interesting luminescent and magnetic properties [1-6]. Discovery of novel polyoxoanions structures from unexpected route can be guidance to understanding of formation mechanism of polyoxometalates. Hill [7] showed that in 0.5 M NaCl and upon slow evaporation, the A-type Keggin sandwich complexes of [(M(OH2)2) (-PW9O34)2]n- [8-9] are converted to [((MOH2)M2PW9O34)2(PW6O26)]17- (M = Mn(II) or Co(II)). The structure consists of two B--(MOH2)M2PW9O343- units joined by a B-type hexavacant PW6O2611- fragment to form a C-shaped polyoxometalate [10]. We have recently reported structurally analogous [(PW10Sm2O38)4(W3O8(H2O)2(OH)4)]22- and proposed an evidence for decomposition of [A-PW9O34)2(H2OSm)3CO3]11- [11]. The structures of these sandwich type polyoxometalates consist of a (MOH2)3CO3 belt encapsulated by two A-PW9O34 groups. The oxygen atoms of the CO32- are sitting at the midpoints of the sides of the triangle formed by the three M(III) ions [12]
