Aquo cations

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... 

In some respects these salts resemble those of iron the aquo-cation [Co(HiO)f,] (pink) occurs in solution and in some solid salts, for... 

A major difficulty in an inorganic text is to strike a balance between a short readable book and a longer, more detailed text which can be used for reference purposes. In reaching what we hope is a reasonable compromise between these two extremes, we acknowledge that both the historical background and industrial processes have been treated very concisely. We must also say that we have not hesitated to simplify complicated reactions or other phenomena—thus, for example, the treatment of amphoterism as a pH-dependent sequence between a simple aquo-cation and a simple hydroxo-anion neglects the presence of more complicated species but enables the phenomena to be adequately understood at this level. 

On the other hand, Cp2TiCl2 and CpTiCl dissolve in boiling dilute hydrochloric acid, yielding aquo cations that retain the Cp groups, eg,... 

Thus in all corrosion reactions one (or more) of the reaction products will be an oxidised form of the metal, aquo cations (e.g. Fe (aq.), Fe (aq.)), aquo anions (e.g. HFeO aq.), Fe04"(aq.)), or solid compounds (e.g. Fe(OH)2, Fej04, Fe3 04-H2 0, Fe203-H20), while the other reaction product (or products) will be the reduced form of the non-metal. Corrosion may be regarded, therefore, as a heterogeneous redox reaction at a metal/non-metal interface in which the metal is oxidised and the non-metal is reduced. In the interaction of a metal with a specific non-metal (or non-metals) under specific environmental conditions, the chemical nature of the non-metal, the chemical and physical properties of the reaction products, and the environmental conditions (temperature, pressure, velocity, viscosity, etc.) will clearly be important in determining the form, extent and rate of the reaction. 

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. 

It is apparent (Fig. 1.21) that at potentials removed from the equilibrium potential see equation 1.30) the rate of charge transfer of (a) silver cations from the metal to the solution (anodic reaction), (b) silver aquo cations from the solution to the metal (cathodic reaction) and (c) electrons through the metallic circuit from anode to cathode, are equal, so that any one may be used to evaluate the rates of the others. The rate is most conveniently determined from the rate of transfer of electrons in the metallic circuit (the current 1) by means of an ammeter, and if / is maintained constant it can eilso be used to eveduate the extent. A more precise method of determining the quantity of charge transferred is the coulometer, in which the extent of a single well-defined reaction is determined accurately, e.g. by the quantity of metal electrodeposited, by the volume of gas evolved, etc. The reaction Ag (aq.) -t- e = Ag is utilised in the silver coulometer, and provides one of the most accurate methods of determining the extent of charge transfer. 

As we shall see (Chapter 4), the kinetics of surface complex formation is often related to the rate of H20 loss from the aquo cation. This is another (indirect) evidence for inner-sphere complex formation. 

Ions in aqueous solution have primary and secondary hydration shells the former can generally be related to the coordination shell about the metal ion in hydrates containing aquo cations (cf. previous section). NMR studies of aqueous solutions containing slow-exchange cations, at low... 

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... 

Most M3+(aq) and a number of M2+(aq) cations behave in much the same way as Fe3+(aq). Evidently the viabilities of simple aquo-cations are restricted by the possibilities for hydrolysis, as well as the redox processes discussed in Section 5.4. The simplest explanation for the hydrolysis of Fe3+(aq) acknowledges the considerable polarising power of Fe3+. When surrounded by six water molecules, the cation will tend to attract electron density from the O atoms. This makes the coordinated O atoms less attractive to protons than those in the bulk solvent, and encourages the transfer of protons from the coordination sphere. If the bonding in the complex [Fe(H20)6]3+ is regarded as covalent, with the formation of coordinate bonds, we can arrive at the same conclusion the delocalisation of the positive charge over the complex ion [Fe(H20)50H]2+ will tend to discourage recombination of the coordinated OH- ion with a proton. 

Where the polarising power of a cation is very great, no simple aquo-cation - or even no cationic species whatever - may be stable to hydrolysis, even at extremely acid pH. For example, let us contemplate the viability of B3+(aq). The hydration enthalpy of B3+ is estimated to be about -6000kJ mol-1. From this and the other relevant data given in the treatment of BF3(s) in Section 5.3, we can estimate AH° for the reaction ... 

When the metal ion of an insoluble salt forms a complex ion, the aquo cation is removed from solution, shifting the solubility equilibrium toward solution species. 

Table 5. Relative shift on water protons for different Ln(III) aquo-cations (Er(III) = 1.0) ...

The major reagents used in the study of both small and large molecules have been either Ln(III) aquo cations or Ln(III)(FOD)3 complexes54. It has been found that Ln(III)(EDTA) complexes are valuable reagents for anionic centres and that... 

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