Thermodynamics of complexation of zinc(II) with chloride, bromide and iodide ions in hexamethylphosphoric triamide

Abstract

The complexation of zinc(II) with chloride, bromide and iodide ions has been studied by calorimetry in hexamethylphosphoric triamide (HMPA) containing 0.1 mol-dm−3 (n-C4H9)4NClO4 as a constant ionic medium at 25°C. The formation of [ZnXn](2−n)+ (n=1,2,3,4 for X=Cl; n=1,2 for X=Br, I) is revealed, and their formation constants, enthalpies and entropies were determined. It is proposed that the zinc(II) ion is fourcoordinated in HMPA and the coordinating HMPA molecules are stepwise replaced with halide ions to form [ZnXn(hmpa)4−n](2−n)+ (n=1−4), as is the case for the cobalt(II) ion. Furthermore, the formation of [ZnClI], [ZnBrI], [ZnBrCl] and [ZnBrCl2]− is revealed in the relevant ternary systems. It is found that the affinity of a given halide ion X− to [ZnCl]+, [ZnBr]+ and [Znl]+ is practically the same.