An X-Ray Diffraction Investigation of the Coordination Structure of Mn(II) Ions in Highly Concentrated Aqueous MnBr2 and MnCl2 Solutions

Abstract

Abstract The solute structures of highly concentrated aqueous 5.087 and 2.744 mol dm−3 MnBr2, and 4.744 and 2.732 mol dm−3 MnCl2 solutions were determined by the analysis of the radial-distribution function curves of X-ray diffraction experiments. Mixed-ligand aquahalogenomanganese(II) complexes were found to be present in these solutions, holding a distorted octahedral geometry. The interatomic distances of the aquabromo complex are 2.178–2.184 Å for Mn–OH2 and 2.596–2.643 Å for Mn–Br; those of the aquachloro complex are 2.183–2.185 Å for Mn–OH2 and 2.492–2.496 Å for Mn–Cl. The interatomic distances of Mn–OH2 are shorter than those of the MnBr2·4H2O and MnCl2·4H2O crystals and are consistent with those of the Mn(OH2)6(NO3)2 crystal. The length of the Mn–Cl bond here determined is practically the same as that within the MnCl2·4H2O crystal, but the Mn–Br bond found is shorter than that within the MnBr2·4H2O crystal. The average coordination number of the Mn–Br contact per Mn(II) increased from 1.104 to 1.212 with an increase in the Br/Mn ratio from 2.076 to 3.269, while the number of the Mn–Cl contact per Mn(II) increased from 1.259 to 1.501 with an increase in the Cl/Mn ratio from 2.000 to 3.352. In all solutions, the aquamonohalogeno and aquadihalogeno complexes exist as the main species, but the aquadihalogeno complex is more dominant in the MnCl2 solution than in the MnBr2 solution.