Acid-base equilibria hydrated metal ions

The overall effect of complex formation is to remove a hydrated metal ion from the mixture of ions in solution by displacing the equilibrium in favour of the complex, cf. the similar process in the formation of water in acid-base titrations and precipitation reactions. 

The released metal ions are covered by a hydration shell, the size of which depends on the valency of the ion and the size of the nucleus. The hydrate complexes [M(H20) ] of multi-valent ions can deprotonate (i.e. they act as acids), which affects the chemical equilibrium of the reaction (3.21). As a result, the apparent solubility is a function of the pH-value. Deprotonation leads to a transformation of the hydrate complexes into hydroxide complexes [M(OH) ] ". The neutral hydroxide complexes are hardly soluble. The total amount of dissolved material and its composition can be calculated from the acid-base-equilibria of the complex ions and the pH. In general, the lower the main group of the metal is, the higher the oxide solubility. 

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