Bronsted Acidity of Metal Ions

The metal ions are found in aqueous solution surrounded by water molecules as aqua complexes (or aqua acids). The hydrated ions can undergo stepwise hydrolysis with delivery of one or more H+ to the bulk solvent. This process occurs due the attraction of the metal ion to the electron cloud of its water hydration, and the weakening of the OH bond in the H2O molecule. The process can be represented by the reaction. 

Any positive ion can participate in this reaction to some extent, as expressed as the equilibrium constant (KJ for the above equation. 

Metal ions with a single positive charge have only one hydrolysis step, while the divalent or trivalent cations undergo stepwise hydrolysis. For comparison only, the first step of hydrolysis, characterized by the pK, is considered for each element. 

Each step in the hydrolysis reduces the charge as hydroxycations are formed. For any step of this reaction, a standard hydrolysis constant can be written  

Finally, a neutral hydrated metal hydroxide is formed. Usually, such neutral species will be insoluble and precipitate. Metal hydroxide usually starts to precipitate when the pH of the solution is approximately equal to the pK of the metal ion (Table 3.11). If the effective electronegativity of the element is high enough, the hydrolysis process can continue to produce hydroxyanions and even oxoanions. This is the case of metal ions in their highest oxidation states, which have extremely high effective electronegativity. For example, Mn(VII) forms the Mn04 oxoanion in 

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