Structure-stability relationships of some meta- and para-hydrazopyrazolone derivatives with trivalent lanthanide ions

Abstract

Acid dissociation constants (p K H) of a series of 10 compounds (I to X) of substituted 5-pyrazolone dyes have been determined potentiometrically in 75% (v/v) dioxane-water at 30 °C and <0.1 M ionic strength. The stability constants of their 1:1 and 1:2 chelates with 13 lanthanide ions Ln(III) have also been determined. The effect of substituents on the hydrazo moiety of the ligand is discussed. Data are correlated and the results are used to explain the stabilization of such chelates by dative π bonding between Ln(III) and the ligand.