Hexaquo cations

The hexaquo-cation occurs in the blue-violet alums, for example NH4V(S04)2.12H20... 

The ions that tend to be involved in AB cements include such species as Al , Mg, Ca and Zn. These are all capable of developing a coordination number of six, and hexaquo cations are known to be formed by each of these metal ions (Huckel, 1950). The typical requirements for an ion to develop such coordination characteristics are that the ion should exist in the -I- 2 or -1-3 oxidation state, and in this state should be of small ionic radius (Greenwood Earnshaw, 1984). 

Figure 10. Comparison of isotopic fractionations determined between Fe(II)aq and Fe carbonates relative to mole fraction of Fe from predictions based on spectroscopic data (Polyakov and Mineev 2000 Schauble et al. 2001), natural samples (Johnson et al. 2003), DIR (Johnson et al. 2004a), and abiotic formation of siderite under equilibrium conditions (Wiesli et al. 2004). Fe(II)aq exists as the hexaquo complex in the study of Wiesli et al. (2004) hexaquo Fe(II) is assumed for the other studies. Total cations normalized to unity, so that end-member siderite is plotted at Xpe = 1.0. Error bars shown reflect reported uncertainties analytical errors for data reported by Johnson et al. (2004a) and Wiesli et al. (2004) are smaller than the size of the symbol. Fractionations measured on bulk carbonate produced by DIR are interpreted to reflect kinetic isotope fractionations, whereas those estimated from partial dissolutions are interpreted to lie closer to those of equilibrium values because they reflect the outer layers of the crystals. Also shown are data for a Ca-bearing DIR experiment, where the bulk solid has a composition of q)proximately Cao.i5Feo.85C03, high-Ca and low-Ca refer to the range measured during partial dissolution studies (Johnson et al. 2004a). Adapted from Johnson et al. (2004a).

The manganese(II) cations are derived from manganese(II) oxide. They form colourless salts, though if the compound contains water of crystallization, and in solutions, they are slightly pink this is due to the presence of the hexaquo-manganate(II) ion, [Mn(H20)6]2+. 

The Effect of Adsorbed Molecules on the Spectrum of the Cu Cations in Zeolites. Figure 5 shows the change in the spectrum corresponding to the transitions between d-electron levels of Cu during dehydration of the zeolite. The spectrum of completely hydrated zeolite revealed a broad absorption band with a maximum at 12,100 cm" Thermal treatment of the zeolite at 100 °C resulted in the appearance of a new absorption band at approximately 15,500 cm" After vacuum treatment at high temperatures, there appeared in the spectrum an absorption band at 11,200 cm" The position of the absorption band due to Cu " in the spectrum of completely hydrated zeolite is close to that of the [Cu(H20)e] complex (12,600 cm" ) (2). This indicates that Cu " enters the hydrated zeolite structure as an octahedral hexaquo-complex. The same conclusion has been reached by other investigators 4, 18, 20) on the basis of e.s.r. spectroscopic measurements of the Cu " cations in completely hydrated zeolites. 

According to a number of authors (28- 1). the Interaction of cations such as Na with a poly-anlon In water can occur under two different modes either a true contact Ion pair Is formed, as dictated by the mass action law and so that the two hydration shells are perturbed or the sodium cation, as the hexaquo species. Is attracted Into the electrostatic potential In the vicinity of the charged counter-Ion It Is like a free Ion In every respect except for being constrjdned not to diffuse outside of a certain volume surrounding the polyelectrolyte. The former Interaction type Is called site binding. The latter, which Is reminiscent of formation of a loose, solvent-separated. Ion pair Is referred to as atmospheric condensation. (Figures 1-2). 

As an example, we consider the equilibrium in aqueous solution between iron hexaquo complex cations and thiocyanate anions on the one hand and the blood red iron thiocyanate complex on the other which can be described in the following simplified manner ... 

Coprecipitation takes place by condensation of hexaquo complexes in solution, forming brucite-like layers where both cations are homogeneously distributed, and with the anions (and water molecules) hosted between the layers. Observation of recently precipitated particles and powder X-ray diffraction (PXRD) studies show that formation of the layers and of the interlayer domains is immediate, without previous delamination of the brucite-like layers [54]. 

---