Aqeous metal ion reaction with ammonia
Theses are two steps when aqua ions react with ammonia. Firstly, it acts as a base, hence, deprotonation occurs:
For M2+ ions Fe2+, Co2+ and Cu2+; and for M3+ ions Al, Cr3+ and Fe3+:
However, with excess NH3, something else occurs in the case of Co2+ and Cr3+ whereby because the NH3 can also act as a ligand, and that it is similar in size to the H2O ligands, it can actually replace some of the H2O ligands - a process called ligand exchange and the precipitate of the hydroxide re-dissolves.
With Co2+ and Cr3+, no change in coordination number occurs:
However, ligand exchange may be incomplete, such as in the case of Cu2+:
Firstly, copper (II) ion is deprotonated into its hydroxide:
Secondly, ligand exchange occurs:
The Cu-O bonds are longer and weaker than the Cu-N bonds, hence, they are stationed above the tetrammine planar.
Ligand exchange can also occur between ligands of different sizes. This leads to a change in coordination number. This occurs with Co2+ and Cu2+:
The chelate effect
Chelation is the formation of complexes with bidentate or multidentate ligands, where more than 1 coordinate bond is formed with the ligand. This forms a more stable complex ion than with than those with unidentate ligands.
Not only does this ligand exchange produce a more stable complex, but it is also entropically driven because it produces more entities on the right than on the left.
Drafted by Eunice (Chemistry)