Water as a Ligand

In aqueous systems, water is the most abundant ligand. Distortion of the electron distribution on a water molecule as a result of coordination causes the protons to be more readily ionizable, as already mentioned. The result may be the formation of hydroxo (HO") or oxo (O2") complexes, a process called hydrolysis. That is, the aqua complex9 [M(H20)x]"+ (x is usually 6) may be partially or even completely deprotonated, depending on the nature of M"+ and the pH. This behavior is illustrated by Fe3+ or Al3+. One has loss of protons in, for example, the following reactions:

[Al(H20)50H]2+ + H2O ^ H3O+ + [Al(H20)4(0H)2]+ (9-51)

Ultimately this may lead to oxo species through loss of two protons from one O, and finally to oxo anions such as [Al03]3" if the medium is basic enough. There may be polymerization—for example, as follows:

2[Al(H20)50H]2+ ^ OH" + [(H20)5Al " (OH) " Al(H20)5]5+ (9-52)

where the hydroxo ligand bridges two metal ions. With further deprotonation and polymerization, repulsion among ligands usually results in a change in coordination number from octahedral 6 to tetrahedral 4. After aging, compounds with compositions such as FeO(OH) and AlO(OH), which have bridging ligands around each metal ion to give a three-dimensional cross-linked structure, result. The reactions are all pH sensitive and often reversible, but once the final highly polymerized products have formed, they may be quite resistant to dissolution upon increase of acidity.

The extent of these hydrolysis reactions can be correlated roughly with size and charge of the metal ion, being more extensive for the smallest and most highly charged ion at a given pH. In general, one has the following:

9It is the convention to write the formula for a complex ion or molecule in square brackets to define the coordination sphere. This practice should not be confused with the use of square brackets to denote concentrations.

1+ ions: form simple aqua complexes over most of the pH range.

2+ ions: aqua complexes predominate in acidic media, but hydroxo species occur in basic media. However, larger alkaline earths remain aqua complexes even in strongly basic media, while very small ions such as Be2+ are hydrolyzed under neutral or mild acidic conditions.

3+ ions: chiefly present as hydroxo complexes in the pH range of natural waters.

4+ ions: most of these form hydroxo or oxo complexes in all but highly acidic solutions.

The highly polymerized hydroxo precipitates that frequently form upon hydrolysis of the 3+ and 4+ ions are often of a gelatinous nature, resulting from the irregular cross-linked network that is built up. These usually rearrange and become more crystalline with time. The formation of such precipitates can be used in flocculation processes for water purification as discussed in Chapter 11.

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