The complex [CuII(H2O)(HNPDA)] (HNPDA = ((2-carboxyethyl)imino)diacetate) has been synthesized and characterized by X-ray diffraction. The title compound crystallizes in the monoclinic system, space group P21/c, with a = 10.559 (3) Å, b = 8.715 (2) Å, c = 11.217 (2) Å, β = 111.85 (3)°, Z = 4, V= 958.1 Å3, and Dmeasd= 1.998 g cm-3. Final R = 0.031 and Rw = 0.029 for 1663 independent observed reflections. Each tetradentate HNPDA2-ligand chelates one copper(II) ion with three nearly coplanar and short bonds (Cu-N = 1.997 (3) Å, Cu-O(13) = 1,963 (3) Å, and Cu-(23) = 1.972 (3) Å) and a fourth longer bond (Cu-O(5) = 2.488 (3) Å), making up an interesting O-aminopropionic acid-copper(II) six-membered chelate ring, which is puckered in a boat-type conformation. The in-plane short bond Cu-O(H2O) = 1.924 (2) Å and a trans-apical long bond Cu-O(14i) = 2.494 (3) Å, involving an O donor atom of the bidentate-bridged acetate arm of a second HNPDA2-ligand (referred to by the superscript i), complete the tetragonally elongated octahedral coordination of each copper(II) ion. Thus, each ligand molecule links two copper atoms and each copper(II) is bonded to two ligands. The 21-axis-related copper complexes share one oxygen atom, thus forming infinite polynuclear zigzag chains, reinforced by O(4)-H(04)⋯O(24iv) (symmetry code iv: x, -1 + y, z) intrachain hydrogen bonds (linking alternating complex entities). In addition, both hydrogen atoms of the coordinated water molecule are involved in interchain hydrogen bonds with two different homologous chains, making up tunnels of irregular hexagonal cross section that run parallel to the [010] axis in the tridimensional network. The potentiometric and conductometric titrations, thermal stability, spectral (IR, reflectance, ESR) properties, and magnetic susceptibility data are interpreted in light of the known crystal structure of this complex. © 1987, American Chemical Society. All rights reserved.