Crystalline Cadmium Iodide Adducts with N,N-Dimethylacetamide and N,N-Dimethylformamide: Formation in Ternary Solution-Solid Systems and Thermochemistry

This work presents new data on the solubility of cadmium iodide in a mixture of N,N-dimethylacetamide (DMA) and N,N-dimethylformamide (DMF), as well as on the composition of crystalline compounds in equilibrium with the saturated solution. The solubility of the salt varies nonlinearly across the entire range of mixed solvent compositions. The solubility isotherm comprises four regions, each corresponding to the crystallization of individual solvates. The crystalline solvates exhibit similar structures and consist of the complex anion [Cd2I6]2- and a complex cation, whose composition depends on the ratio of the components in the binary solvent. The cation includes either one of the amides or both in a 1:2 ratio, [Cd(DMA)4(DMF)2]2+ and [Cd(DMA)2(DMF)4]2+. Standard heats of dissolution in water were determined for both solvents and all solvates. From these data, the standard enthalpies of formation of the solvates were calculated. The enthalpies of formation of the solvates correlate with the molar ratio of amides in the complex cation and exhibit a direct dependence on the donor number of the amide.