CHEM5 - Inorganic Compounds in Aqueous Solution

Lewis Base - Lone Pair Donor
Lewis Acid - Lone Pair Acceptor

- Metal cations become hydrated in water to form metal-aqua complex ions. [M(H2O)6]n

NH3(aq) + BF3(aq) > NH3BF3(aq)

A ligand acts as a Lewis Base and the central metal ion acts as a Lewis Acid.
An acidity/hydrolysis reaction is a reaction between the metal-aqua ion and a base.
E.g/ [Fe(H2O)6]2+(aq) + H2O(l) <> [Fe(H2O)5(OH)]+(aq) + H3O+(aq)

The metal-aqua 2+ ions release H+ ions, so an acidic solution is formed. However, there is only slight dissociation so it is weakly acidic.
Metal-aqua 3+ ions form more acidic solutions than 2+ ions because:
- They're small with a big charge so have a high charge density.
- Therefore, they're more polarising so attract electrons from O of H2O more strongly and will release H+ ions more readily.

- Adding OH- ions to hexaaqua complexes produces insoluble metal hydroxides (precipitates)
[M(H2O)6]2+(aq) + 2OH- > [M(H2O)4(OH)2](s) + 2H2O
[M(H2O)6]3+(aq) + 3OH- > [M(H2O)3(OH)3](s) + 3H2O

- All metal hydroxide precipitates will dissolve in acid.
- Some are amphoteric: Act as both acids and bases. Therefore they will dissolve in an excess of base as well as acids. (i.e Al3+ and Cr3+)

NH3 can also be added to form precipitates.
- With excess NH3, the metal ions form ammine complexes with Cu2+, Co2+ and Cr3+.
[Cu(H2O)6]2+(aq) + 6NH3 > [Cu(NH3)6]2+(aq) + 6H2O
[Co(H2O)6]2+(aq) + 6NH3 > [Co(NH3)6]2+(aq) + 6H2O
[Cr(H2O)6]3+(aq) + 6NH3 > [Co(NH3)6]3+(aq) + 6H2O

Metal 2+ ions react with Na2CO3 to form insoluble metal carbonates.
[M(H2O)6]2+(aq) + CO3 2-(aq) <> MCO3(s) + 6H2O(l)

But Metal3+ ions are stronger acids, so will form precipitates.
2[M(H2O)6]3+(aq) + 3CO3 2-(aq) > 2[M(H2O)3(OH)3](s) + 3CO2(g) + 3H2O(l)

- The stability of a species depends upon pH of the solution. A solution with less oxygen per metal ion will be stable in acidic solution. A solution with more oxygen will be stable in alkaline solution.
Eg/ 2CrO4 2-(aq) + 2H+(aq) <> Cr2O7 2-(aq) + H2O(l)
Cr2O7 2-(aq) + 2OH- (aq) <> 2CrO4 2-(aq) + H2O(l)

Ligand Exchange

- The replacement of one ligand by another in a complex.
- Ligand coordination number effects the energy of the d-orbitals.
- When NH3 replaces H2O, the coordination number doesn't change as they're both uncharged, small ligands. [M(H2O)6]3+ + 6NH3 <> [M(NH3)6]3+ + 6H2O.
- NH3 is acting as a Lewis Base.

With [Co(H2O)4(OH)2] the substitution is complete and all six ligands are replaced. However, with [Cu(H2O)4(OH)2] the substitution is incomplete and only 4 ligands are replaced. [Cu(H2O)2(NH3)4]2+

Ligand Exchange by Chloride Ions

- In the presence of concentrated Hydrochloric Acid, Chloride ions can replace the water ligands in some complex ions and form an anionic complex.

- Large, negatively charged Chloride ions will repel one another more than smaller, neutral water/ammonia ligands. Therefore fewer of them can pack around the central metal cation.
Eg/ Hexaaquacopper(II) [Cu(H2O)6]2+
Tetrachlorocuprate(II) [CuCl4]2-

- When chloride ions replace water ligands the coordination number changes from 6 to 4. This causes an increase in entropy as the number of species increases from 5 to 7.
- However, the reaction is endothermic because more bonds (6) are broken than formed (4) and the chloride ligands form weaker bonds than the water ligands.
- These reactions are generally readily reversible and the addition of water to the solution will cause the chloride ligands to be replaced by water ligands. This reaction is used in the test for water; Blue Cobalt Chloride paper turns pink in the presence of water.

Ligand Exchange by Polydentate Ligands

- The substitution of a unidentate ligand with a bidentate/multidentate ligand leads to a more stable complex. (Bidentate - NH2CH2CH2NH2 / C2O4 2-) (Multidentate - EDTA 4-)

[Fe(H2O)6]3+(aq) + 3C2O4 2-(aq) <> [Fe(C2O4)3]3-(aq) + 6H2O
[Cu(H2O)6]2+(aq) + EDTA 4-(aq) <> [Cu(EDTA)]2-(aq) + 6H2O
- These reactions are favoured because they cause an increase in entropy. There are always more species on the right of the equation than the left. The fact that multidentate complexes are more stable than monodentate complexes is known as the chelating effect. (which is used to treat metal poisoning)

Enthalpy Changes in Ligand Substitution

[Co(NH3)6]3+(aq) + 3(en) > [Co(en)3]3+(aq) + 6NH3
There are 6 Co-NH3 bonds broken, and 6 Co-en bonds formed.
Enthalpy change is always about 0 because the same number and type of bonds are broken and formed.

Haemoglobin

- Is the molecule that causes blood to appear red. It carries oxygen from the lungs to cells in the body. Haemoglobin contains an Fe2+ ion which forms a haem complex where the iron is bonded to 4 Nitrogen atoms in a plane within a large organic molecule called porphyrin.

- Octahedral coordination of the iron involves a 5th Nitrogen atom, below this plane, from a protein called globin, with the 6th position, above the plane, occupied by either molecular oxygen or the oxygen atom of a water molecule. Oxygen is a poor ligand that is easily released to cells, where its concentration is low and ligands that can form stronger bonds with the Fe2+ ion, such as CO, bind irreversibly and destroy Haemoglobins ability to carry Oxygen. These substances are toxic.

Chlorophyll

- Is the pigment responsible for the green colour in plants and is structurally similar to haem. However, the porphyrin ring in chlorophyll is built around Mg rather than Fe2+.