X-ray scattering measurements have been carried out for concentrated aqueous solutions of divalent transition-metal perchlorates of manganese(II) to zinc(II). The radial distribution curves showed that each central metal ion was octahedrally surrounded by six water molecules at the distances of 2.20 Å for Mn2+–OH2, 2.12 Å for Fe2+–OH2, 2.08 Å for Co2+–OH2, 2.04 Å for Ni2+–OH2, and 2.08 Å for Zn2+–OH2. The results from the radial distribution curves were confirmed by the direct analysis of reduced intensities. Concentrated aqueous solutions of copper(II) and zinc(II) sulfates have also been measured. The bond distances of 1.94 Å for the Cu2+–OH2 (equatorial), 2.38 Å for the Cu2+–OH2 (axial) and 2.08 Å for Zn2+–OH2 were consistent with those obtained for the perchlorate solutions. No evidence for the contact ion pairs of metal and sulfate ions was found by the present X-ray measurements. The bond energies between metal ions including copper ion were discussed by application of the Garrick model with a slight modification.

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The structure of zinc chloride complexes with different ratios of chloride to zinc, formed in concentrated ZnCl2 aqueous solutions, were determined from large-angle X-ray scattering using concentrations of the chloride complexes estimated by complementary Raman spectroscopic measurements. The highest chloro complex, [ZnCl4]2-, is tetrahedral with a Zn-Cl bond length of 2.294(4) Å. The trichloro complex, [ZnCl3]-, which coordinates one water molecule, is pyramidal with the Cl-Zn-Cl angle 111°. The Zn-Cl and the Zn-H2O bonds are 2.282(4) and 1.9 Å, respectively. The two lower complexes, [ZnCl2] and [ZnCl]+ , cannot be separated by Raman spectra. The average Zn-Cl distance in these complexes is 2.24 Å, and the average Zn-H2O distance is 1.9 Å. In [Zn(H2O)6]2+ the Zn-H2O distance is 2.15 Å.

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