Effect of the pH of the medium on the structure of gadolinium-containing complexes according to EPR data

Dilute and concentrated aqueous solutions of gadolinium acetate and chlorid e were studied by EPR in a wide range of pH. It was shown that, at pH < 2, the compositions of gadolinium aqua ions land probably of other rare earth metals) in dilute and concentrated solutions are identical: [Gd(OH2)(8)](3). At higher pH, this complex was found to hydrolyze to aqua hydrocomplexes . Experiments showed that the crystal field acting on Gd3+ in [Gd(OH2)(8)]( 3+) is weak. This result was explained under the assumptions that water mol ecules are exchanged between aqua complexes and the solvent or that the cub ic coordination polyhedral is nonrigid (i.e., the O-h <-> D-4d transitions occur).