THERMODYNAMICS OF COMPLEXATION OF ZINC(II) WITH CHLORIDE, BROMIDE AND IODIDE-IONS IN HEXAMETHYLPHOSPHORIC TRIAMIDE

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-degrees-C. The formation of [ZnX(n)](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 four-coordinated in HMPA and the coordinating HMPA molecules are stepwise replaced with halide ions to form [ZnX(n)(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 [ZnI]+ is practically the same.