Two novel 3-D coordination polymers,Tb2[C6(COO)6](H2O)6(l)and Pr2[C6(COO)6](H2O)6(2),have been hydrothermally synthesized from mellitic acid (benzenehexacarboxylic acid C6(COOH)6)、Tb(NO3)3 (or Pr(NO3)3)、NaOH and H20,and characterized by Fluorescence analysis,TG,IR spectrum and X-ray crystallography.The two crystals of complexes are both belong to orthorhombic space group Pnnm,with the complex (1) a=13.414(3)(A),b=6.5858(1)(A),c=10.117(2)(A),V=893.8(3)(A)3,Z=4,F(000)=716.0,Ooof=1.034,Final R indices [l>2sigma(Ⅰ)]..R 1 =0.0186,wR2=0.043 1 ; the compound (2) a=13.5492(2)(A),b=6.70960(1)(A),c=10.24630(1)(A),V=931.49(2)(A)3,Z=4,F(000)=692.0,Goof=1.044,Final R indices [l>2sigma(Ⅰ)]:R1=0.0158,wR2=0.0362.The complexes (1) and (2) are two different rare earth mellitate,which have the same structure.Fig.l shows their molecular structure.The Tb or Pr cations are coordinated by six carboxylic oxygen atoms of mellitate anions with composition of [C6COO)6]6-and three water oxygen atoms.Each mellitate anion linking eight neighboring Tb or Pr,and four carboxyls of mellitate anion are bidentate chelated coordinated like Ol and O2,which distorted trigonal prisms produces a two-dimensional layered structure parallel to (101); the other two carboxyls are bidentate bridging coordinated to metal cations like O3 and O4.The carboxylate group coordinated to Tb or Pr cations serve as a hexadentate ligand in that structure.The number of water molecules incorporated into the mellitate compound is controlled mainly by ionic radii of metal cations in the structure.Furthermore,the ionic radii of metal cations in the mellitate compound play an essential role in arrangement of mellitate anions in the structure,whether as a one-dimensional infinite chain,or a two-dimensional layered structure,or a three-dimensional framework structure.The sheets are further connected by hydrogen bonding interaction to yield a three-dimensional (3-D) structure.