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This paper deals with the structure of the YIII complex with aminopolycarboxylic acids, synthesis and structural determination of the complex (NH4)[YIII(edta)(H2O)3]3H2O (edta = ethylenediaminetetraacetic acid). The crystal and molecular structures of the (NH4)[YIII(edta)(H2O)3]3H2O complex have been determined by single-crystal X-ray structure analysis. The crystal of the complex (NH4)[YIII(edta)(H2O)3]3H2O belongs to orthorhombic crystal system and fdd2 space group. The crystal data are as follows: a = 1.944 1(9) nm, b = 3.545 9(18) nm, c = 1.219 6(6) nm, V = 8.407(7) nm3, Z = 16, Mr = 503.25, Dc = 1.590 gcm 3, = 2.844 mm 1 and F(000) = 4 160. The final R and Rw are 0.048 6 and 0.133 2 for 3 388 (I>2 (I)) unique reflections, respectively. The complex anion [YIII(edta)(H2O)3] has a pseudo-monocapped square antiprismatic nine-coordinate structure in which the six coordinate atoms (two N and four O) are from an edta ligand and three water molecules coordinate to the central YIII ion directly. From the results it can be predicted that the YIII ion can also form a nine-coordinate complex with ttha (=triethylenetetraminehexaacetic acid) ligand, so the radioactive complex anion [90YIII(Httha)]2 can supply a free carboxyl group ( CH2COOH) being used for molecular embellishment to form the TDDS for the treatment of cancers.