THE EFFECT OF NON-COORDINATED CHARGED GROUPS ON THE STABILITY OF COMPLEXES IN AQUEOUS SOLUTION. THE STABILITY OF COMPLEXES OF 2,3-DIHYDROXYNAPHTHALENE-6-SULFONIC ACID

Abstract

Formation constants are determined for Ca²⁺, Mg²⁺, Cd²⁺, Co²⁺, Ni²⁺, La³⁺, Cu²⁺, UO₂ ²⁺, Al³⁺, Th⁴⁺, Fe³⁺, with the ligand DHNSA (2,3-dihydroxynaphthalene-6-sulfonic acid). It is shown that for chelating oxygen donor ligands, plots of log K₁ for the ligand forming a complex with metal ions gives an LFER (linear free energy relationship), when plotted against log K₁ (OH⁻) for the metal ions, which has an intercept of log 55.5, in agreement with previous proposals on the origin of the chelate effect. It is shown that adding sulfonic acid groups to such chelating oxygen donor ligands, as in DHNSA, gives an LFER which has an intercept much higher than log 55.5. The source of this extra stabilisation is discussed, and it is concluded that in a ligand such as DHNSA, the two phenolic oxygens are coordinated to the metal ion, while the sulfonic acid groups stabilize the complex by an outer-sphere electrostatic attraction.