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Acidity of Transition Metal Complexes
Acidity of Transition Metal Complexes
- If we measure the pH of the solutions of a variety of metal ions we will find that these ions act as weak acids when in solution.
- The aluminum ion is an example.
- When aluminum nitrate dissolves in water, the aluminum ion reacts with water to give a hydrated aluminum ion:
Al+3(aq) = Al(H2O)6+3
- What this means is that the aluminum ion has the strongest interactions with the six closest water molecules (the so-called first solvation shell).
Al+3(aq) + 6H2O → Al(H2O)6+3
- We frequently see the formula of this ion simply as “Al3+(aq)”, without explicitly noting the six water molecules that are the closest ones to the aluminum ion and just describing the ion as being solvated in water (hydrated).
- This is similar to the simplification of the formula of the hydronium ion, H3O+ to H+.
- However, in this case, the hydrated aluminum ion is a weak acid and donates a proton to a water molecule.
- Thus, the hydration becomes important and we may use formulas that show the extent of hydration:
Al(H2O)6+3(aq) + H2O(l) ⇋ Al(H2O)5(OH)+2(aq) + H3O+(aq)
As with other polyprotic acids, the hydrated aluminum ion ionizes in stages, as shown by:
Al(H2O)6+3(aq) + H2O(l) ⇋ Al(H2O)5(OH)+2(aq) + H3O+(aq)
Al(H2O)5(OH)+2(aq) + H2O(l) ⇋ Al(H2O)4(OH)2+1(aq) + H3O+(aq)
Al(H2O)4(OH)2+1(aq) + H2O(l) ⇋ Al(H2O)3(OH)3(s) + H3O+(aq)
As you can see in each stage the Hydrated ion Loses 1 water and gains a hydroxide - in actual fact the water loses a proton
Also - Note that some of these aluminum species are exhibiting amphiprotic behavior, since they are acting as acids when they appear on the left side of the equilibrium expressions and as bases when they appear on the right side.
However, the ionization of a cation carrying more than one charge is usually not extensive beyond the first stage.
This means we can compare the Ka of the first stages of ionisation of different transition metal complexes to compare their relative acid strength.
+3 ions have a stronger effect on water then +2 ions so +3 ions are more acidic this is because the Central metal ion has a stronger polarising effect on the water - making it easier for a Proton to be released.
The key learning points here are that:
- Hydrated metal complexes can behave as weak acids
- +3 ions are more acidic than +2 ions as they hold on to the OH- more strongly
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