The Aqua Ion and Hydrated Salts

For many years it was assumed that, like the succeeding d-block metals, scandium formed a [Sc(H20)6] ion. Various pieces of evidence indicated this was not the case - salts containing dimeric [(H20)5Sc(0H)2Sc(H20)5] + ions with approximately pentagonal 

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Nd(H20)9] + ions in Nd(Br03)3.9H20, but is significantly different from that of the 8-coordinate Nd + in the hydrated sulfate and chloride salts. This was important evidence in favour of the aqua ion being 9 coordinate. The spectrum of Nd + in concentrated HCl is significantly different, and suggests that an 8-coordinate species is present here. 

The properties of the aqua-ions [Fe(H20)6] " and [Fe(H20)6] + in solution have been placed in context and reviewed in detail. Their structures, estabhshed in the solid state in several X-ray diffraction studies of hydrated iron(ll) and iron(lll) salts, have been investigated in solution by... 

Using O-labelled water, it is possible to confirm the hydration number in [Mg(H20)6] by use of NMR spectroscopy, but for the aqua ions of Ca +, Sr + and Ba, exchange between coordinated water and solvent is too fast on the NMR timescale for such studies. An octahedral coordination sphere has been established for [Mg(H20)g] in a number of crystalline salts, and also for [Ca(H20)g] +. The [Mg(H20)g] + ion dissociates to some extent in aqueous solution, but hydrated cations of the later metals are not appreciably ionized and solutions of their salts derived from strong acids are neutral. 

The aqua ions (M = Al, Ga, In, Tl) are acidic (see equation 6.34) and the acidity increases down the group. Solutions of their salts are appreciably hydrolysed and salts of weak acids (e.g. carbonates and cyanides) cannot exist in aqueous solution. Solution NMR spectroscopic studies show that in acidic media, Al(III) is present as octahedral [Al(H20)g], but raising the pH leads to the formation of polymeric species such as hydrated [Al2(OH)2] and [Al7(OH)ig] +. Further increase in pH causes Al(OH)3 to precipitate, and in alkaline solution, the aluminate anions [A1(0H)4] (tetrahedral) and [Al(OH)g] (octahedral) and polymeric species such as [(H0)3Al(p-0)Al(0H)3] are present. The aqueous solution chemistry of Ga(III) resembles that of Al(III), but the later metals are not amphoteric (see Section 12.7). 

Partial hydrolysis of the aqua ion produces a bridged dimeric species, the [(H20)5Sc(0H)2Sc(H20)5]" + ion, which has been found in three salts, [(H20)5Sc(/u.-0H)2Sc(H20)s] X4 2H2O (X = Cl, Br), and also a benzene sulfonate [(H20)5Sc(/x-0H)2Sc(H20)5] (C6H5S03)4-4H20. In view of the tendency of many of the hydrated lanthanide halides to contain coordinated halide ions, the chlorides in particular, the absence of halide from the coordination sphere of scandium in these compounds is remarkable. 

The trigonal-bipyramidal pentaaquo zinc species has been structurally characterized332 IR spectroscopy has been utilized in studies of the hydration of zinc in aqueous solution and in the hydrated perchlorate salt aqua ions Zn(H20)62. 9... 

To summarize, calcium ions in the solution can be present as hydrated Ca2+ions, and CaHEDTA and CaEDTA2 complexes, of which only the positive calcium aqua complexes (Ca2+) participate in the ion-exchange reaction. In the process, calcium ions dissolve from montmorillonite to the solution, and mostly hydrogen ions get into the interlayer space of montmorillonite. In addition, sodium ions are also present in the system (EDTA is added as disodium salt H4EDTA is hardly soluble in water), which also affects the ion-exchange process. The ratio of cCa aCa can be plotted as a function of the concentration of Ca2+ (Figure 2.11). 

The only aqua ions are Ag+(aq) and Ag2+(aq). However, the former binds H20 molecules only weakly, and the crystalline salts of the Ag+ ion are rarely, if ever, hydrated. By contrast the Au+ ion does not exist except in complexes it is exceedingly unstable with respect to the disproportionation... 

Hydroxide, aqua, and hydrates. From the similar absorption spectra of Am in aqueous solution, AmCls, and in Lads, and the linear relationship between the decay rate of the americium fluorescence and the number of inner-sphere water molecules, it has been concluded that Am " is coordinated by nine inner-sphere water molecules. Similarly, the hydration number for the Cm ion has been estimated to be nine on the basis of fluorescence lifetimes. EXAFS studies of aqueous Am and Cm, however, have suggested coordination numbers closer to 10. " EXAFS investigation of Cf " " in aqueous solution indicates a coordination number of 8.5 ( 1.5), with a Cf—O distances of 2.41 0.02 A. This coordination number was confirmed for Am in the solid state by isolation of single crystals of the triflate salt of nonaqua complex, which contains a tricapped, trigonal prismatic cation that is isostructural with the analogous Pu" compound. ... 

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