This study has investigated the reaction of A--SiW9O3410- with M = Ce3+, Pr3+, Nd3+, Sm3+, Eu3+ and Gd3+ in aqueous Na2CO3 solution and found that sandwich type polyoxometalates of [(-SiW9O34)2(H2OM)3CO3]13- are formed only with Eu3+ and Gd3+. The complexes were synthesized at room temperature and isolated as K+/Na+ salt, but single crystal structure determination was carried out on (NH4)12Na[(-SiW9O34)2(H2OGd)3CO3]16.5H2O (CH87Gd3N12NaO90.50Si2W18). The complex crystallizes in triclinic system, space group P-1 with a = 11.997 Å, b = 19.618 Å, c = 21.619 Å, = 64.332°, = 82.876°, = 86.992° and Z = 2. The crystal structure of the complex consist of two lacunary A--SiW9O3410- Keggin moieties which are linked by a (H2OGd)3CO3 belt. The results prove the retention of the isomeric form of the starting lacunary anion A--SiW9O3410- upon the preparation of the complexes. The title new complexes are less stable than of [(-SiW9O34)2(H2OM)3CO3]13- M = Y3+ and Yb3+ analogue because of the larger ionic radii of Eu3+ and Gd3+.
