Complexes are atoms or more ions or compounds in which an ion is surrounded by a number of molecules or oppositely charged ions called ligands.
The ligands are joined to the central atom or ion by co-ordinate bonds. The pair of electrons been donated by the ligand. The donor atoms are usually oxygen, nitrogen and halogens.
The transition elements with vacant 'd' orbitals readily form complexes but, is not limited to these elements.
The most common ligands in complexes are:
1. Water: most ions form hydrated ions in solution.
For example: [Cu(H20)6]2+ hexaaqua copper (II) ion
2. Ammonia: the co-ordinate bond is formed between the nitrogen atom and the metal ion, the nitrogen atom donating the lone pair of electrons.
For example: [Co(NH3)6]3+ hexaammine colbalt (III) ion.
3. The Cyanide ion: the lone pair of carbon atoms is used to form co-ordinate bonds.
For example: [Fe(CN)6]4- hexacyano ferrate (II) ion
4. The Halide ion: the halide provides the lone pair of electrons.
For example: [CuCl4]3- tetrachloro cuprate (II) ion
- Number ligands using mono, di, tri, tetra, penta, hexa.
- Identify ligands using names ending in -o for anions. For example: F- fluoro, CN- cyano, Cl- chloro, OH- hydroxo (H2O is aqua, NH3 is ammine).
- Name the cation using English name in +ve complex but latinized name in -ve complex with suffix -ate. For example: aluminate, plumbate, cuprate, ferrate, zincate, nickelate.
- Indicate oxidation number of central cation using I, II, III etc.
These reactions typically involve the replacement of a ligand by another ligand.
Ligand replacement reactions.
A solution of copper (II) sulphate is blue. This solution contains hydrated Cu2+ ions. The addition of ammonia changes the solution to a dark blue containing the ion tetraammine copper (II).
The reaction can be summarised as shown below:
[Cu(H20)6]2+(aq) + 4NH3(aq) → [Cu(NH3)4(H20)2]2+(aq) + 4H20(l)
When potassium thiocyanate is added to a pale yellow solution of iron (III) ions, a deep blood red solution is formed, containing the ion pentaaquathiocyanato iron (III).
[Fe(H20)6]3+(aq) + SCN-(aq) → [Cu(SCN)(H2O)5]2+(aq) + H2O(l)
There are many more examples of ligand replacement (or exchange) reactions. You should check with your syllabus the ones you need to learn.