AS/A-Level Chemistry - Substitution reactions

After knowing ligand and transition metal, let's see how they react and substitute each other in A-Level Chemistry!

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.

For example:

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.

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Drafted by Eunice (Chemistry)

Reference

https://www.youtube.com/watch?v=RM6l2JfHxg8