CHLORO AND BROMO COMPLEXATION OF THE MANGANESE(II) ION AND SOLVATION STRUCTURE OF THE MANGANESE(II), IRON(II), COBALT(II), NICKEL(II), COPPER(II), AND ZINC(II) IONS IN HEXAMETHYLPHOSPHORIC TRIAMIDE

The complexation of the manganese(II) ion with chloride and bromide ions has been studied by calorimetry in hexamethylphosphoric triamide (HMPA) containing 0.1 mol dm(-3) ((n)-C4H9)(4)NClO4 as a constant ionic medium at 25 degrees C. The formation of binary [MnX(n)]((2-n)+) (n = 1-3 for X = Cl; n = 1, 2 for X = Br) and of ternary [MnClBr] and [MnCl2Br](-) is revealed, and their formation constants, reaction enthalpies, and entropies were determined. These thermodynamic data, especially reaction entropies, suggest that the manganese(II) ion has a solvation number larger than four in HMPA, though transition metal(II) ions are usually thought to be four-coordinated due to bulkiness of an HMPA molecule. Indeed, our fluorescent EXAFS (extended X-ray absorption fine structure) study showed that the manganese(II) ion is five-coordinated in HMPA. On the other hand, it is revealed that the other divalent transition metal ions such as iron(II), cobalt(II), nickel(II), copper(II), and zinc(II) are all four-coordinated,