Cd(ll) complexes with phthalic acid: solution study and crystal structure of cadmium(ll)

The acid-base properties of phthalic acid (H,L) have been determined by simultaneous spectroscopic, potentiometric, and polarographic measurements. Stability constants of the complexes CdHLf, CdL, CdL,'-, C~L;-, C~H,L; and their distribution versus pH were determined at 25OC and ionic strength in 0.1 M NaNO,. The advantages of each method are pointed out in such a way as to describe more precisely the interactions between cadmium and phthalic acid over a wide range of ligand and metal concentrations. The crystal structure of the (CdL) complex has been established by single-crystal X-ray diffraction. Cd(C,H,O,).H,O is orthorhombic, space group Pbca, a = 23.463(5), b = 9.293(2), c = 7.730(1) A, Z= 8, and p = 2.323 g cm-,. The structure was refined to a final R value of 0.054 for 323 1 unique reflections with I > 3u(4. The crystal is a three-dimensional polymeric network in which the Cd atom is seven-coordinated. The first ligand coordinates in a bidentate carboxylate (0,O') mode (Cd-0 = 2.372(5) and 2.445(5) A); a second ligand is part of a seven-membered ring formed with its two carboxylate groups (Cd-0 = 2.317(5) and 2.341(5) A); two other phthalic ligands make one coordination bond (Cd-0 = 2.255(5) and 2.449(5) A) while the seventh bond is a water molecule (Cd-0 = 2.333(6) A).