Cations in aqueous solution

Figure A2.4.3. The localized structure of a hydrated metal cation in aqueous solution (the metal ion being assumed to have a primary hydration number of six). From [5].

Table 14.2 shows that all three elements have remarkably low melting points and boiling points—an indication of the weak metallic bonding, especially notable in mercury. The low heat of atomisation of the latter element compensates to some extent its higher ionisation energies, so that, in practice, all the elements of this group can form cations in aqueous solution or in hydrated salts anhydrous mercuryfll) compounds are generally covalent. 

Another phenomenon that is closely associated with acid-base equilibria is the so-called hydrolysis of metal cations in aqueous solution, which is probably better considered as the protolysis of hydrated cations, e.g. ... 

Similar analyses of 2-methyl-, 2,3-dimethyl-, 2,3,6-trimethyl-, and 2,3,6,7-tetramethyl-l,4,5,8-tetraazanaphthalene showed that all the neutral species were anhydrous. Whereas the cations (in aqueous solution) of the 2-methyl and 2,3-dimethyl derivatives were predominantly ( > 95%) hydrated, the 2,3,6-trimethyl cation had only a trace of hydrated species and that of the 2,3,6,7-tetramethyl derivative was anhydrous. ... 

Metal cations in aqueous solution often form chemical bonds to anions or neutral molecules that have lone pairs of electrons. A silver cation, for example, can associate with two ammonia molecules to form a silver-ammonia complex ... 

Fonseca, M.G. and Airoldi C. (2000) Mercaptopropyl magnesium phyllosilicate-thermodynamic data on the interaction with divalent cations in aqueous solution. Thermochimica Acta, 359, 1-9. 

Cations in aqueous solutions have an effective radius that is approximately 75 pm larger than the crystallographic radii. The value of 75 pm is approximately the radius of a water molecule. It can be shown that the heat of hydration of cations should be a linear function of Z /r where is the effective ionic radius and Z is the charge on the ion. Using the ionic radii shown in Table 7.4 and hydration enthalpies shown in Table 7.7, test the validity of this relationship. 

Some cations in aqueous solution, such as Na+, K+, and Ba2+, undergo no real significant reaction with the water molecules in a solution. This is because their acid strengths are much less than the acid strength of water. In other words, these cations are such weak acids that they do not react with water. Because of this, when these cations are dissolved in water, the pH is unchanged. 

Ferrocene-mediated enzyme sensors have usually been obtained by adsorption of the mediator onto the electrode surface because of this insolubility in aqueous solution. Due to the good solubility of ferrecinium cations in aqueous solution complications arise from leakage of the mediator from the electrode surface. 

Neutron diffraction examination of CsFe (S04)2 12H20 and CsFe (Se04)2 12H20 at 15 K showed a small but significant difference between Fe—O bond distances, 1.994(1) A in the sulfate, 2.002(1) A in the selenate. This difference is attributed to the different tilt an s of the coordinated water, 0.6° and 18.6°, respectively. A XRD structure determination on CsFe (8004)2 I2H2O gave Fe—0= 1.989(4) A (at room temperature). This selenate has the a-alum structure, whereas the corresponding sulfate has the /3-alum structure. These results have been placed in the context of X-ray and neutron diffraction studies of structures of hydrated cations in aqueous solution. ... 

Although POMs have been known for about 200 years [6d,e,f] a large number of novel polyoxoanions with unexpected shapes and sizes are still being discovered. However, despite this diversity that precludes from complete rational and systematic design of synthesis for all POMs, a few guidelines can be retained. For specific details, the reader is referred to books, reviews, and original papers [3-7, 27]. Primarily, the evolution of aquated metal cations in aqueous solution depends on the pH of the medium, going from the hydrated cation [M(H20)x] , viable in very acidic medium, to the oxoanion [MOx] - obtained in very basic solution (see Sch. lb). The route between these two extremes is populated by a series of more or less stable oxocations and hydroxocations. 

Amyes, T. L. et al.. Formation and stability of N-heterocyclic carbenes in water The carbon acid pK(a) of imidazolium cations in aqueous solution. J. Am. Chem. 

Table 7.2 Examples of first-row transition element aqua cations in aqueous solution...

Baciocchi et al43 have reported the existence of the pH-dependent mechanistic dichotomy for the deprotonation of 4-methoxybenzyl alcohol radical cation in aqueous solution. In neutral and acidic solutions the 4-MeOC6H4CH2OH + radical cation undergoes C-H deprotonation, while in basic solution (pH 10), the reaction is initiated by deprotonation of the OH group. DFT calculations were carried out and reveal that the OH induced O-H deprotonation is consistent with the charge controlled reaction, while the C-H deprotonation, observed when the base is HjO, appears to be effected by frontier orbital interactions43. 

Many reducing cations in aqueous solutions can photoreduce water in low yields when irradiated in their charge transfer bands. Similarly, oxidizing cations can photo-oxidize water. Several hydride complexes of transition metals such as cobalt decompose on irradiation to yield molecular hydrogen... 

The earlier discussion of complexes of indium(III) with nitrogen ligands emphasized the importance of six-coordination, and in keeping with this, XH and U5In NMR studies have identified [In(H20)6]3+ as the cation in aqueous solutions of perchloric acid.103,104 Confirmatory evidence has been obtained from dilatometric105 and X-ray diffraction106 methods. Proton... 

To E. C. Franklin, these compounds are merely the analogues of hydrates which are more stable than ordinary hydrates. It is probable that the metallic cations in aqueous solutions are also complexes with water molecules but not so stable as with ammonia. 

Electron transfer from the excited states of Fe(II) to the H30 f cation in aqueous solutions of H2S04 which results in the formation of Fe(III) and of H atoms has been studied by Korolev and Bazhin [36, 37]. The quantum yield of the formation of Fe(III) in 5.5 M H2S04 at 77 K has been found to be only two times smaller than at room temperature. Photo-oxidation of Fe(II) is also observed at 4.2 K. The actual very weak dependence of the efficiency of Fe(II) photo-oxidation on temperature points to the tunneling mechanism of this process [36, 37]. Bazhin and Korolev [38], have made a detailed theoretical analysis in terms of the theory of radiationless transitions of the mechanism of electron transfer from the excited ions Fe(II) to H30 1 in solutions. In this work a simple way is suggested for an a priori estimation of the maximum possible distance, RmSiX, of tunneling between a donor and an acceptor in solid matrices. This method is based on taking into account the dependence... 

Table 8 Values of 10D

The quantum yield of the [Ru(bpy)3]2 + photosensitized reduction of Co(III)-Schiff base complex cation in aqueous solution is greately affected by the composition of the polymer-supported polyanionic donors such as vinylbenzylamine-iV,iV-diacetate-... 

Reactions of cations in aqueous solution with sodium hydroxide - Al3+, Fe2+, Fe3+, Cu2+, Ca2+, Zn2, Cr3+,. An excess of sodium hydroxide solution is added slowly to a small volume of the solution containing the cation. This is an exercise in observation, organisational and recording skills and in the ability to write chemical equations - word, molecular and ionic. 

Baciocchi E, Bietti M, Steenken S (1999) Kinetic and product studies on the side-chain fragmentation of 1-arylalkanol radical cations in aqueous solution oxygen versus carbon acidity. Chem EurJ 5 1785-1793... 

Behrens G, Bothe E, Koltzenburg G, Schulte-Frohlinde D (1980) Formation and structure of 1,1-di-alkoxyalkene radical cations in aqueous solution. An in situ electron spin resonance and pulse conductivity study. J Chem Soc Perkin Trans 2 883-889... 

Freiberg M, Meyerstein D (1980) Reactions of aliphatic free radicals with copper cations in aqueous solution, part 2. Reactions with cupric ions a pulse radiolysis study. J Chem Soc Faraday Trans 176 1825-1837... 

Asmus K-D, Gobi M, Hiller K-O, Mahling S, Monig J (1985) S.. N and S.. O three-electron-bonded radicals and radical cations in aqueous solutions. J Chem Soc Perkin Trans 2 641-646 Bacq ZM (1965) Chemical protection against ionizing radiation. Thomas, Springfield Batt L, Burrows JP, Robinson GN (1981) On the isomerization of the methoxy radical - relevance to atmospheric chemistry and combustion. Chem Phys Lett 78 467-470... 

Shi Y, Fluang C, Wang W, Kang J, Yao S, Lin N, Zheng R (2000a) Electron transfer from purine de-oxynucleotides to deoxynudeotides deprotonated radical cations in aqueous solution. Radiat... 

The remainder of this chapter is concerned with the stabilities of ions (mainly cations) in aqueous solution, with respect to oxidation, reduction and disproportionation. Ions in solution are surrounded by solvent molecules, oriented so as to maximise ion-dipole attraction (although there may be appreciable covalency as well). The hydration number of an ion in aqueous solution is not always easy to determine experimentally it is known to be six for most cations, but may be as low as four for small cations of low charge (e.g. Li+) or as high as eight or nine for larger cations (e.g. La3+). 

We now look at some examples of redox reactions involving simple cations in aqueous solution. Electrochemical terminology will often be encountered, since e.m.f. measurements on electrochemical cells are important sources of thermodynamic data in this area. For example, the reduction potential ° for the half-reaction ... 

One of the factors which governs the viability of a cation in aqueous solution is the susceptibility of water to oxidation ... 

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