EXAFS STUDIES OF AQUEOUS-SOLUTIONS OF MANGANESE(II) CHLORIDE AND BROMIDE - STRUCTURAL VARIATIONS WITH CONCENTRATION AND INTERACTIONS WITH SOLVENT

EXAFS data, collected at the SRS at Daresbury Laboratory, for aqueous solutions of MnBr2 and MnCl2 at a variety of concentrations were analysed by least-squares curve fitting calculations. The data, obtained at ambient temperature, were found to be consistent with equilibria between species of the type [MnX(6-n)(H2O)n](n-4)+ (where X = Cl, Br and n = 4, 5, 6) for which it was possible to determine the average value of n (i.e. n(av)) at each concentration. To fit the data, a third distance, in addition to Mn-X and Mn-O in the complexes, was shown to be significant in the least-squares calculations; this third distance at 4.4 angstrom represents the radius of the first hydration sphere around the complexes and is reasonably interpreted as Mn to solvent water molecules hydrogen bonded to the water molecules in the complexes. By carefully parameterising the calculations using EXAFS data for solid reference compounds, the dependence of n(av) on concentration was determined; calculations were carried out both with data sets for single concentrations and also with data sets for four different concentrations simultaneously. The latter calculations enabled the direct refinement of the slope k and the intercept M0 for the concentration dependence represented as n(av) = M0 - kc/mol dm-3, where c is the concentration. For both MnCl2 and MnBr2 solutions, M0, the number of water ligands in the complex at infinite dilution, was found to be 6, as expected. The slopes k were found to be 0.18(1) and 0.25(1), respectively. The greater slope in the case of MnBr2 shows that its solutions have a smaller value of n(av) than found in comparable MnCl2 solutions; in neither case does n(av) reach 4 before saturation occurs.