TARTRATE COMPLEXES OF THE RARE-EARTH ELEMENTS II. THE dl- AND meso-TARTRATE COMPLEXES OF La, Ce, Pm, Tm, AND Y

Abstract

The liquid-liquid distribution method has been used to measure the stability constants of the dl-tartrate complexes of La3+, Ce3+, Pm3+, Tm3+, and Y3+, and the meso-tartrate complexes of Ce3+ and Tm3+. Thenoyltrifluoroacetone was the organic phase extractant, and the aqueous phase, acetate buffered (pH = 4.53), was maintained at a constant ionic strength (0.0597) by means of sodium perchlorate.The order of stabilities for the two stages of ion association are[Formula: see text][Formula: see text]The b1/b2 values, where b is a stepwise stability constant defined as [Formula: see text]are lower (by factors of 4–8) for the dl-tartrates than for the meso-tartrate complexes. The b1/b2 ratios decrease as the ionic radius of the metal ion decreases, and for the dl-tartrates of Tb3+, Y3+, and Tm3+, the b1/b2 values are 5.3, 5.0, and 4.3 respectively. The enhanced stabilities of the 1:2 (ML2−) dl-tartrate chelates have been interpreted in terms of ligand–ligand hydrogen bridging in the 1:2 complex.