The first three stepwise formation constants of the 2-hydroxy- 3- methylbutanoate (isopropylglycolate), 2, 3-dimethyl- 2-hydroxybutanoate (methylisopropylglycolate), 3, 3-dimethyl-2-hydroxybutanoate (tertiarybutylglycolate), and 2-hydroxy-2, 3, 3-trimethylbutanoate (methyltertiarybutylglycolate) complexes of the trivalent rare earths and yttrium were determined at 25° C and an ionic strength of 0. l molar (NaCl04 ). The stabilities of the complexes followed the ionic radius trend quite regularly. The stabilities generally decreased with increasing complexity of the alkyl substituents on the ligand, but replacing a hydrogen atom by a methyl group invariably enhanced the stabilities of the complexes, especially in the case of the heavier, smaller rare-earth ions. It was postulated that the stabilities of the heavier rare-earth complexes with the methyl-substituted homologues were enhanced because of a smaller, more favorable hydroxyl-carboxyl bond angle.
Paul, George S. and Powell, Jack E., "Stability constants of the trivalent lanthanons with some alpha-hydroxycarboxylic acids" (1965). Ames Laboratory Technical Reports. 97.