Structure of Concentrated Aqueous Solutions of Alkaline Earth Metal Chlorides and Bromides

Abstract The magnesium, calcium, strontium and barium ions bind six, eight, eight and nine water molecules in the first hydration shell, and ca. 12, 16, 16 and 18 water molecules in the second one, in dilute aqueous solution, respectively. With increasing concentration of alkaline earth metal halides, water molecules in the second hydration shell are exchanged for hydrated chloride and bromide ions forming solvent shared ion–pairs. At high concentrations the coordination number of the calcium and strontium ions decreases to seven in aqueous solution, while the magnesium and barium ions maintain their octahedral and nine–coordinate configuration, respectively. In the case of highly concentrated calcium chloride solutions a di– or polymeric complex with a double aqua bridge is formed. The inner–coordination shell of water molecules remains in solid magnesium halide hydrates precipitated from aqueous solution. Dimeric and polymeric aqua complexes of hydrated calcium, strontium and barium halides form in the solid state. However, except for highly concentrated calcium chloride solutions, such complexes have not been detected in aqueous solution. Graphical Abstract