Spectrophotometric determination of stability constants of anti-stepwise complexes

Abstract

There is a considerable knowledge of chemical equilibria of the stepwise formation of complexes in coordination chemistry. However, the coordination equilibria of complex formation of the type, M + R ⇌ MR ⇌ M₂R ⇌ M₃R ⇌…⇌ M_nR is less well understood. As their compositions are contrary to the formation of stepwise complexes, they are called ‘anti-stepwise complexes’. The existence of such complexes is not unusual and often causes difficulties in speciation analyses. Therefore, it is essential to investigate further the chemical equilibria in solution. In this paper, based on Bjerrum's function (n) and the method of corresponding solutions, a method is suggested for determining the stability constants of this kind of complex. For demonstration purposes it has been used with the iron(III) and Eriochrome Cyanine R system and satisfactory results were obtained. This method is generally applicable and could be used for most ‘anti-stepwise’ systems.