Amphoteric oxides and hydroxides

It is instructive to compare acid strengths of hexaaqua ions with other acids. The values of MeC02H (eq. 7.9) and HOCl (eq. 7.15) are similar to that of [Al(OH2)6l 

The facile acid dissociation of [Fe(OH2)e] means that its aqueous solutions must be stabilized by the addition of acid, which (by Le Chatelier s principle) drives equilibrium 7.36 to the left-hand side. 

Proton loss is, in some cases, accompanied by the formation of dinuclear or polynuclear species in aqueous solution. For example, after the dissociation of H from [Cr(OH2)g], the product undergoes an intermolecular condensation (eq. 7.38). The resulting dichromium species (Fig. 7.7) contains bridging hydroxy groups. 

A similar reaction occurs in the corresponding V(III) system. On going from V(III) to V(IV), the charge density on the vanadium centre increases. As a result, the dissociation of two protons from one coordinated H2O occurs, and the blue oxidovanadium(IV) ion, 7.9, is formed. It is common for this cation to be written simply as [VO]2+, even though this is not a naked vanadium oxido species. 

The oxides and hydroxides of the metals of Group 3 and higher tend to be only weakly basic, and most display an amphoteric nature. Most of these compounds are so slightly soluble in water that their acidic or basic character is only obvious in their reactions with strong acids or bases. 

In general, these compounds tend to be more basic than acidic thus the oxides and hydroxides of 

However, in concentrated hydroxide solutions, jugate bases of the oxide or hydroxide these substances form anionic species which are the con-  

These are called the aluminate, zincate, and ferrate ions. Other products, in which only some of the 

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Like other metal oxides and hydroxides, amphoteric spedes dissolve in addic solutions because their anions, 0 or OH , react with acids. What makes amphoteric oxides and hydroxides special, though, is that they also dissolve in strongly basic solutions. This behavior results from the formation of complex anions containing several (typically four) hydroxides bound to the metal ion (T FIGURE 17.21) ... 

The solubility of metal salts is also affected by the presence of certain Lewis bases that react with metal ions to form stable complex ions. Complex-ion formation in aqueous solution involves the displacement by Lewis bases (such as NH3 and CN ) of water molecules attached to the metal ion. The extent to which such complex formation occurs is expressed quantitatively by the formation constant for the complex ion. Amphoteric oxides and hydroxides are those that are only slightly soluble in water but dissolve on addition of either acid or hase. 

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