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,