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Potentiometric, 1 H, 31 P NMR spectroscopic and X-ray studies were carried out to investigate the complex formation of Al() with iminodiacetic acid (IDA), nitrilotriacetic acid (NTA), their mixed carboxylic-phosphonic and purely phosphonic derivatives. The stability constants of the complexes formed were determined at 25 ЊC and at 0.2 mol dm Ϫ3 ionic strength (KCl). It was found that substitution of CO 2 Ϫ by PO 3 2Ϫ increases the overall stability of the complexes, due to the higher basicity of the phosphonic groups. However, the higher spatial requirement of the phosphonic moiety and the greater electrostatic repulsion between the dinegatively charged PO 3 2Ϫ moieties overcompensate this effect, resulting in a somewhat weaker metal binding capacity for the phosphonic derivatives. According to the 1 H NMR spectra of Al() complexes of IDA, complex formation renders the two protons of each CH 2 group inequivalent, while the CH 2 groups of NTA and its derivatives remain chemically and magnetically equivalent in their Al() complexes. As no symmetrical arrangements of the donor atoms in their Al() complexes can be expected with most of the ligands, the rate of intramolecular rearrangement motions of the binding functional groups seems to be different for the IDA and the NTA derivatives. DALTON