The Relative Stability of Internal Metal Complexes. I. Complexes of 8-Hydroxyquinoline, Salicylaldehyde, and Acetylacetone

The stability constants of a series of internal metal complexes derived from 8-hydroxyquinoline, salicylaldehyde, and acetylacetone have been determined by means of an electrometric method. A quantitative treatment of the chelation of 8-hydroxyquinoline, based on the theory of step equilibria, is given and the modifications required for dealing with the other substances investigated are indicated. Two important facts emerge from this survey : (1) The order of the metals (as regards stability of their complexes) is substantially the same for each of the three chelating substances studied. (2) The order of the stability of the complexes derived from the three chelating substances is independent of the nature of the coordinating metal, the 8-hydroxyquinoline complex of a metal being more stable than its acetylacetone complex which in turn is more stable than its salicylaldehyde complex. Measurements were made of the stability constants in water-dioxan mixtures and the effect of changing the proportions of the two components in the solvent mixture was studied in several instances. The values of stability constants of internal metal complexes in water-dioxan mixtures are greater than those in water. It is shown that insolubility of internal metal complexes is not, in general, a criterion of their relative stability. The possible significance of stability-constants in chemotherapy, biochemistry, and analytical chemistry is briefly discussed.