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Crystalline samples of CuH3L · 3.5H2O (L = anion of N,N,N′,N′-tetrakis(carboxymethyl)-2,4-diamino-butyric acid; H5TDB = H5L) have been characterized by X-ray diffraction and other methods. The compound crystallizes in the monoclinic system, space group P21/n (a = 6.260(2), b = 18.505(5) c = 15.659(3) A, β = 91.78(3)° and Z = 4). Final R = 0.049 and Rw = 0.054 for 1400 independent observed reflections. The structure consists of one-dimensional chains of CuH3L complex units and non-coordinated water molecules linked together in a network of hydrogen bonds. Only two [H(062) and H(N2)] of the three “acid” hydrogen atoms of the ligand (H3L2−) have been located in the structure. Each H3L2− anion acts as a tetradentate chelating agent for a Cu(II) ion by means of the C-substituted hydrogen nitrilotriacetato (NTA) group (Cu-N(1) 2.026(9), Cu-O(11) 1.971(8), Cu-O(81) 2.003(8) and Cu-O(61) 2.186(6)A. The O(11) and O(61) atoms are part of carboxylato and carboxylic groups, respectively. The distorted five c...