Affinity of lanthanoid(III) ions for nitrogen-donor ligands in aqueous solution

Abstract

A model of the chelate effect is used to predict that lanthanoid(III) ions should form chelated complexes with glycine at pH >6, and bipyridyl complexes resistant to hydrolysis in aqueous solution with logK₁∼ 1. The values of logK(ML)= 3.26, logK[ML(OH)]= 8.79, and logK[ML(H)]= 10.43 have been obtained in 0.15 mol dm^–3 NaClO₄ at 25 °C for M = Nd^III and L = glycine. At 25 °C in 0.5 mol dm^–3 Na[NO₃], logK₁(bipyridyl) has been determined for the following lanthanoid(III) ions: La, 0.8; Ce, 0.9; Pr, 0.9; Nd, 0.9; Sm, 0.9; Gd, 0.8; Dy, 0.9. Ultraviolet spectroscopy and n.m.r. line-broadening experiments are used to support the potentiometric studies, which indicate that, in addition to the complexes bound through the carboxylate to protonated glycine, ring closure involving bonding to the nitrogen occurs at higher pH.