Structural Studies on Saturated Aqueous Solutions of Manganese(II), Cobalt(II), and Nickel(II) Chlorides by X-ray Diffraction

摘要

As a part of our studies on crystallization processes of electrolytes, the structure of aqueous solutions of MCl_2 (M = Mn, Ca, Ni) equilbrated with hydrate crystals, MCl_2 centre dot mH_2O (m = 6, 4,2), was investigated by means of X-ray diffraction at 25, 40, 55, and 70 deg C. The complexes formed in MnCl_2 solutions, were found to be mixed-ligand chloroaqua octahedral complexes of M~(2+) ions with the Mn-O and Mn-Cl distances of about 220 and 251 pm, respectively. The average number of Mn-Cl and Mn-O interactions increased from 1.2 to 1.9 and decreased from 4.8 to 4.1, respectively, with changing MnCl_2 solutions from Mn25 (MnCl_2 solution at 25 deg C) to Mn70 (MnCl_2 solution at 70 deg C). In the octahedral species of Co~(2+), the Co-O and Co-Cl distances were found to be about 211 and 240 pm, respectively. With an increase in the saturated concentration by changing temperature from 25 to 70 deg C, the average coordination number of the Co-Cl contact per Co~(2+) increased from 0.5 to 1.2, and the average number of Co-O interactions decreased from 5.5 to 4.8. The structural analysis was carried out by taking into consideration the existence of the tetrahedral species in the solutions saturated at 40, 55, and 70 min C, on the assumption of the existence of [CoCl_4]~(2-). The Co-Cl distance was found to be 228 pm, while the number of Co-Cl interactions in the [CoCl_4] complex was calculated to be 3.7 by the least-squares calculations. The Ni-O and Ni-Cl distances were estimated to be about 206 and 237 pm, respectively. The frequency factor n of the Ni-O and Ni-Cl interactions decreased monotonously from 5.6 to 5.0 and increased from 0.4 to 1.0, respectively, with increasing NiCl_2 concentration. The n values of the Co-Cl and Ni-Cl interactions of the octahedral complexes increased sharply with