Unit 5 Section 4 Ligand Substitution

If the ligands are of similar size then the coordination number of the complex ion doesn't change, and neither does the shape.

H2O and NH3 ligands are similarly sized and are both uncharged.

This means that H2O ligands can be exchanged with NH3 ligands without any change in coordination number or shape.

There will still be a colour change due to a change of ligand.

[Co(H2O)6]^2+(aq) + 6NH3(aq) >>> [Co(NH3)6]^2+(aq) + 6H2O(l)

In a chromium-aqua complex, the H2O ligands can be exchanged with OH- ligands without any change in coordination number or shape because the ligands are similar sizes.

[Cr(H2O)6]^3+(aq) + 6OH^-(aq) >>> [Cr(OH)6]^3-(aq) + 6H2O(l) 

In copper-aqua and cobalt-aqua complexes, the H2O ligands can be exchanged with Cl- ligands, usually from HCl.

The shape of the complex changes from octehedral to tetrahedral because fewer of the Cl- can fit around the central metal ion.

There is also a colour change during this reaction:

Cu is pale blue to yellow-green.

[Cu(H2O)6]^2+(aq) + 4Cl-(aq) <<<>>> [CuCl4]^2-(aq) + 6H2O(l)

Co is pink to blue.

[Co(H2O)6]^2+(aq) + 4Cl-(aq) <<<>>> [CoCl4]^2-(aq) + 6H2O(l)

Sometimes the substitution is only partial - not all of the six H2O ligands are substituted.

E.g. In a copper-aqua complex

• Four of the H2O are substituted with NH3 ligands.
• The shape of the complex changes from  octrehedral to elongated octehedral.
• The colour of solution changes from pale blue to deep blue.

[Cu(H2O)6]^2+(aq) + 4NH3(aq) >>> [Cu(NH3)4(H2O)2]^2+(aq) + 4H2O(l)

E.g. In an iron(III)-aqua complex

• One H2O ligand is exchanged with a SCN- ligand
• Shape changes from octehedral to distorted octehedral
• Colour of solution changes from yellow to blood red

[Fe(H2O)6]^3+(aq) + SCN-(aq) >>> [Fe(H2O)5(SCN)]^2+(aq) + H2O(l)

Ligand exchange reactions can be easily reversed unless the new ion complex formed is more stable than the original complex.

E.g.

• CN- ions form stronger coordinate bonds than H2O molecules with Fe^3+ ions.
• Complex formed with CN- ions is more stable. So is harder to reverse.

[Fe(H2O)6]^3+(aq) + 6CN-(aq) >>> [Fe(CN)6]^3-(aq) + 6H2O(l)

E.g.

• Complexes that contain bidentate ligands are more stable than those that contain H2O ligands. So is harder to reverse.

[Cu(H2O)6]^2+(aq) + 3NH2CH2CH2NH2(aq) >>> [Cu(NH2CH2CH2NH2)3]^2+(aq) + 6H2O(l) 

When unidentate ligands are substituted with bidentate or multidentate ligands, the number of particles increases - the more particles, the greater the entropy.

Reactions that result in an increase in entropy are more likely to occur.

So thats why multidentate ligands always form much more stable complexes than unidentate ligands.

This is known as the chelate effect.

When the hexadentate ligand EDTA^4- replaces unidentate or bidentate ligands, the complex formed is a lot more stable.

It is difficult to reverse these reactions because reversing them would cause a decrease in the systems entropy.