Three oxalate copper(II) complexes, [Cu(bipy)(C2O4)(H2O]. 2H(2)O (1), [Cu(nphen)(C2O4)(H2O)]. 2H(2)O (2). and [Cu(phen)(C2O4)(H2O)].H2O (3) (bipy = 2,2 ' -bipyridine, nphen = 5-nitro-1,10-phenanthroline and phen = 1,10-phenanthroline), have been synthesized and their crystal structures have been determined. Compound 1 crystallizes in the triclinic space group P1 with a = 7.2554(10) Angstrom, b = 10.5712(14) Angstrom, 10.8178(15) Angstrom, alpha = 62.086(2)degrees, beta = 77.478(3)degrees. gamma = 81.773(3)degrees and Z = 2. Compound 2 crystallizes in the triclinic space group P1 with a = 9.582(2) Angstrom, b = 10.086(2) Angstrom, c = 10.592(2) Angstrom, alpha = 64.18(3)degrees, beta = 79.47(3)degrees, gamma = 60.06(3)degrees, and Z = 2. Compound 3 crystallizes in the monoclinic space group P2(1)/n with a = 8.4655(7) Angstrom, b = 9.7057(8) Angstrom. c = 17.4572(14) Angstrom; beta = 103.865(2)degrees, and Z = 4. The crystal structures of all complexes consist of neutral [Cu(L)(C2O4)(H2O)] (L = bipy, nphen, and phen) units and one or two lattice water molecules: in the unit cell. Each copper atom in 1, 2, and 3 involves a five-coordinate CuN2O2O ' environment, with a distorted square-pyramidal structure. In 1 and 2, two lattice water molecules are around each unit of [CuL(C2O4)(H2O)] (L = bipy and nphen) and form two-dimensional networks. Only one lattice water molecule is found in the unit cell of 3 and the two-dimensional structure is different from 1 and 2. The extended three-dimensional structure is formed through pi-pi interactions between layers. The influences of hydrogen bonds and the sizes and Lewis basicity of ligands to the structures were discussed.