Oxidation of aquo cations

The rate constants for the reduction of aquo cations by hydrated electrons and for the oxidation of aquo cations by hydroxyl radicals at 25°C are given in Tables 7.25b and 7.25c, respectively. 

Second-order rate constants for oxidation of aquo cations by hydroxyl radicals in aqueous solution at 25°Ca. 

Table 1.7 shows typical half reactions for the oxidation of a metal M in aqueous solutions with the formation of aquo cations, solid hydroxides or aquo anions. The equilibrium potential for each half reaction can be evaluated from the chemical potentials of the species involved see Appendix 20.2) and it should be noted that there is no difference thermodynamically between equations 2(a) and 2(b) nor between 3(a) and 3(b) when account is taken of the chemical potentials of the different species involved. 

Discuss (a) the acidity and (b) the substitution reactions of metal hexa-aquo cations. [MfH O) ]" (where n = 2 or 3), giving two examples of each type of reaction. Discuss the effect upon the stabilities of the -t- 2 and -f- 3 oxidation states of... 

The acidic properties of coordinated water in aquo cations vary enormously with the cation. Table 10 contains p KX values,19-226 where KX = [M(OH)(OH2 ) r,1)+][H+ (aq)]/M(OH2 )nx+ ][H20], with [H20] taken as unity.7 There is an approximate correlation with electrostatics (charges and ionic radii), but such properties as oxidizing power and softness complicate the pattern.227... 

In aqueous solutions transition metals are characterised by their ability to provide aquo-cations in varying oxidation states and, as a convention, we can regard those oxidation states that are stable in water as being normal . Further, many of the metals can be stabilised in the solid state or in non-aqueous media in oxidation states in which they would reduce or oxidise water and we can, again as a convention, regard these oxidation states as being unusual . 

The first transition metal cation which is unstable in water but which can be generated as a stable entity in HF was U3+ [30]. It was formed by oxidation of the metal by protons in a BF3-HF solution which is non-oxidising and relatively weakly acidic. The UV-vis spectrum of the lilac-colored solution was virtually identical with that observed for an acidified aqueous solution in which the uranium solution was under continuous electrolytic reduction to maintain U(III) as the aquo-cation. 

Figure 6.4. Hydrolysis of metal ions, (a) Predominant pH range for the occurrence of aquo, hydroxo, hydroxo-oxo, and 0x0 complexes for various oxidation states. The scheme attempts to show a useful generalization, but many elements cannot be properly placed in this simplified diagram because other factors, such as radius and those related to electron distribution, have to be considered in inteipreting the acidity of metal ions, (b) The linear dependence of the log,o of the first hydrolysis constant Ki = MOH H / M on the ratio of the charge to the M-O distance (z/d) for four groups of cations (25°C). (Note change of abcissa zero for different groups.) (From Baes and Mesmer, 1976.) (c) Hydrolysis constants of some important metal ions.

Plausibly there is a correlation of the solubility of the stable oxide, hydroxide, or oxyhydroxide of a cation with the stability of the first hydrolysis product (Figure 6. 7). Many multivalent hydrous oxides are amphoteric because of the acid-base equilibria involved in the hydrolysis reactions of aquo metal ions. Alkalimetric or acidimetric titration curves for hydrous metal oxides provide a quantitative explanation for the manner in which the chaige of the hydrous oxide depends on the pH of the medium. The amphoteric behavior of solid metal hydroxides becomes evident from such titration curves. From an operational point of view, such hydrous oxides can be compared with amphoteric polyelectrolytes and can be considered hydrated solid electrolytes, fn -... 

These ligands, with the recently established HjO, have a very close relation with solvent water. Therefore throughout this section chemistry in solution and in the solid state will be juxtaposed and related in a way not possible for other ligands. Indeed the pH dependence of species present in solution has an important bearing on solid complexes, in that the latter are often prepared from solution. The parent complexes, aquo ions [M(OH2)i]" , are important both in salt hydrates and in solution, and have even been studied to a limited extent in the gas phase. Other complexes containing water, hydroxide and oxide in various proportions are all related to these aquo cations by simple and rapidly established proton transfer equilibria. Eventually deprotonation leads to 0x0 anions. Some of these, such as chromate or permanganate, can be considered as complexes others, such as PbOj" and GeOj, are on the borderline with mixed metal oxides. Most such species have... 

It is well-known that aquo cations of heavy elements in the iii oxidation state or higher readily lose protons to form hydroxo complexes. Subsequent condensation reactions between the hydroxo complexes can then form polynuclear species in which the metal ions are linked through hydroxo (M-OH-M) or oxo (M-O-M) bridges. For the formation of polynuclear species, the pH range is critical at too low a pH the ion will exist as the simple aquocation, and at too high a pH the hydrous oxide or hydroxide precipitates. The actinide ions with oxidation number IV are particularly prone to hydrolysis and polymerization, but the v and vi oxo... 

The elements of small formal charge ( = 1,2) do not polarize oxygen very strongly. They form aquo, cations in a large domain of acidity. The oxides of such elements are ionic and exhibit basic behavior in water ... 

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