The hydration of the alkali metal ions in aqueous solution has been studied by large angle X-ray scattering (LAXS) and double difference infrared spectroscopy (DDIR). The structures of the dimethyl sulfoxide solvated alkali metal ions in solution have been determined to support the studies in aqueous solution. The results of the LAXS and DDIR measurements show that the sodium, potassium, rubidium and cesium ions all are weakly hydrated with only a single shell of water molecules. The smaller lithium ion is more strongly hydrated, most probably with a second hydration shell present. The influence of the rubidium and cesium ions on the water structure was found to be very weak, and it was not possible to quantify this effect in a reliable way due to insufficient separation of the O–D stretching bands of partially deuterated water bound to these metal ions and the O–D stretching bands of the bulk water. Aqueous solutions of sodium, potassium and cesium iodide and cesium and lithium hydroxide have been studied by LAXS and M–O bond distances have been determined fairly accurately except for lithium. However, the number of water molecules binding to the alkali metal ions is very difficult to determine from the LAXS measurements as the number of distances and the temperature factor are strongly correlated. A thorough analysis of M–O bond distances in solid alkali metal compounds with ligands binding through oxygen has been made from available structure databases. There is relatively strong correlation between M–O bond distances and coordination numbers also for the alkali metal ions even though the M–O interactions are weak and the number of complexes of potassium, rubidium and cesium with well-defined coordination geometry is very small. The mean M–O bond distance in the hydrated sodium, potassium, rubidium and cesium ions in aqueous solution have been determined to be 2.43(2), 2.81(1), 2.98(1) and 3.07(1) Å, which corresponds to six-, seven-, eight- and eight-coordination. These coordination numbers are supported by the linear relationship of the hydration enthalpies and the M–O bond distances. This correlation indicates that the hydrated lithium ion is four-coordinate in aqueous solution. New ionic radii are proposed for four- and six-coordinate lithium(I), 0.60 and 0.79 Å, respectively, as well as for five- and six-coordinate sodium(I), 1.02 and 1.07 Å, respectively. The ionic radii for six- and seven-coordinate K⁺, 1.38 and 1.46 Å, respectively, and eight-coordinate Rb⁺ and Cs⁺, 1.64 and 1.73 Å, respectively, are confirmed from previous studies. The M–O bond distances in dimethyl sulfoxide solvated sodium, potassium, rubidium and cesium ions in solution are very similar to those observed in aqueous solution.

The hydration reaction is defined as the transfer of an ion or neutral chemical species from the gaseous phase into water, Mⁿ⁺(g) → Mⁿ⁺(aq). In this process, water molecules bind to metal ions through ion-dipole bonds of mainly electrostatic character. The hydration reaction is always strongly exothermic with increasing heat of hydration with increasing charge density of the ion. The structures of the hydrated metal ions in aqueous solution display a variety of configurations depending on the size and electronic properties of the metal ion. The basic configurations of hydrated metal ions in aqueous solution are tetrahedral, octahedral, square antiprismatic, and tricapped trigonal prismatic. This paper gives an overview of the structures of hydrated metal ions in aqueous solution with special emphasis on those with a non-regular coordination figure. Metal ions without d-electrons in the valance shell form regular aqua complexes with a coordination figure, allowing a maximum number of water molecules to be clustered around the metal ion. This number is dependent on the ratio of the metal ion radius to the atomic radius of oxygen in a coordinated water molecule (1.34 Å). The lighter lanthanoid(III) ions have a regular tricapped trigonal prismatic configuration with the M–O distance to the capping water molecules somewhat longer than to the prismatic ones. However, with increasing atomic number of the lanthanoid(III) ions, an increasing distortion of the capping water molecules is observed, resulting in a partial loss of water molecules in the capping positions for the heaviest lanthanoids. Metal ions with d⁴ and d⁹ valance shell electron configuration, as chromium(II) and copper(II), respectively, have Jahn–Teller distorted aqua complexes. Metal ions with low charge and ability to form strong covalent bonds, as silver(I), mercury(II), palladium(II), and platinum(II), often display distorted coordination figures due to the second-order Jahn–Teller effect. Metal ions with d¹⁰s² valence shell electron configuration may have a stereochemically active lone electron pair (hemi-directed complexes) or an inactive one (holo-directed). The hydrated tin(II), lead(II), and thallium(I) ions are hemi-directed in aqueous solution, while the hydrated bismuth(III) ion is holo-directed. The structures of the hydrated cationic oxo-metal ions are reported as well.

Regular octahedral coordination has been reported for some copper(II) complexes in the solid state on the basis of crystallographic studies, e.g. hexaaquacopper(II) bromate, [Cu(OH₂)₆](BrO₃)₂, hexaaquacopper(II) hexafluorosilicate, [Cu(OH₂)₆]SiF₆, and hexakis(pyridine-1-oxide)copper(II) perchlorate, [Cu(ONC₅H₅)₆](ClO₄)₂. These results are not consistent with the elongated octahedral configuration expected from the Jahn–Teller theorem for the d⁹ copper(II) ion nor, in some cases, with results from electron spin resonance studies. The present lattice-independent EXAFS study confirms that the local structure in the copper(II) complexes mentioned above is, in all cases, consistent with a Jahn–Teller induced elongation. Mean equatorial and axial Cu–O bond distances of 1.96(1) and 2.32(2) Å, and 1.95(1) and 2.27(3) Å, were obtained for the hexaaquacopper(II) ions in the bromate and hexafluorosilicate salts, respectively. For the hexakis(pyridine-1-oxide)copper(II) perchlorate only the equatorial mean Cu–O bond distance of 1.96(1) Å could be observed. Evidently, there is orientational disorder of the tetragonally elongated octahedral complexes resulting in too high crystallographic space group symmetry and copper sites in apparently regular coordination geometry. For the hydrated copper(II) ion in aqueous solution, five- and six-coordinated models with different geometries have been evaluated by means of EXAFS and large angle X-ray diffraction (LAXS) data. The combined results are consistent with a Jahn–Teller elongated octahedral configuration with Cu–O_eq 1.95(1) Å, Cu–O_ax 2.29(3) Å, and a distinct second hydration sphere with about eight water molecules and a mean Cu⋯O_II distance of 4.17(3) Å. In dimethylsulfoxide solution EXAFS and LAXS methods show the solvated copper(II) ions to have mean equatorial and axial Cu–O bond distances of 1.96(1) and 2.24(2) Å, respectively. As a model compound for the EXAFS studies, the crystal structure of hexakis(dimethylsulfoxide)copper(II) perchlorate dimethylsulfoxide (1/2), [Cu(OS(CH₃)₂)₆](ClO₄)₂·2(CH₃)₂SO, was determined.