Complexation Reactions in Freshwaters

In this section, as a starting point, it is assumed that all species in solution are in hydrated form, e.g. M H20)6, where the six water molecules form the first co-ordination sphere of the metal ion, The hydrated form is often represented as, for example, M(a ). 

1 Outer and Inner Sphere Complexes. Outer sphere complexation involves interactions between metal ions and other solute species in which the co-ordinated water of the metal ion and/or the other solute species are retained. For example, the initial step in the formation of ion pairs, where ions of opposite charge approach within a critical distance and are then held together by coulombic attractive forces, is described as outer sphere complex formation. 

Inner sphere complexation involves interactions between metal ions and other species in solution which possess lone pairs of electrons. Inner sphere complexation involves the transfer of at least one lone pair of electrons. Those species which possess electron lone pairs are termed ligands and reactions may involve inorganic or organic ligands. 

2 Hydrolysis. In aqueous systems, hydrolysis reactions are an important example of inner sphere complexation for many metal ions. The interaction between hydrated metal ions and water can be written as a series of equilibria for which the equilibrium constants are denoted 

Hydrolysis and, more generally, complex formation equilibria may be described by cumulative stability constants, Using the Equations (3.28)-(3.31) 

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