Chemistry Notes

28.5 Ligand Exchange and Stability Constants

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Ligand Exchange

Ligand Exchange

  • Ligand Exchange (or ligand substitution) is when one ligand in a complex is replaced by another.
  • This happens if the new complex formed is more stable than the original.
  • The ligands in the original complex can be partially or entirely substituted by others.

Changes in Coordination Number and Geometry

  • There are no changes in coordination number, or the geometry of the complex if the ligands are of a similar size.
  • But if the ligands are of a different size, a change in coordination number and the geometry of the complex will occur.

Key Ligand Exchange Reactions

Complexes

Info

  • When a transition element ion is in aqueous solution, it can be assumed to exist in a hexaaqua complex ion (i.e. It has six water ligands attached to it)
  • For example, exists as complex ion.
  • is blue in colour.

Dropwise Addition of or (NOT LIGAND EXCHANGE!)

Reactant:

Reagent: or added dropwise.

Observations: Pale blue precipitate.

Complex Formed: (Neutral and Insoluble)

Type of Reaction: Acid-Base / Deprotonation / Precipitation

Reactions:

  • With

  • With

Explanation Acid-Base Mechanism

  • The acts as a Brønsted-Lowry acid (proton donor).
  • The acts as a Brønsted-Lowry base (proton acceptor).
  • The removes directly from two of the water ligands.

No Ligand Exchange

  • The in the product ligand is not from the added , it is what remains after was removed from two of the ligands.
  • The produced is not from the original complex, it was produced by the neutralisation of the added and the donated by the complex.

Reaction of

  • The first reacts with water to form :

Solubility

  • The original complex was soluble because it had a +2 charge, meaning that it could form strong ion-dipole bonds with water molecules.
  • The resulting complex is uncharged, meaning that it cannot form ion-dipole bonds effectively. It is therefore insoluble.

Addition of in Excess (Ligand Exchange (Partial))

Reactant:

Reagent: Excess

Observations: Light blue precipitate dissolves to give A dark blue solution

Complex Formed:

Type of Reaction: Ligand Exchange ( and )

Reaction:

Explanation Reactant before substitution:

  • The reactant was

What got substituted:

  • The two ligands got substituted by two ligands.
  • Two of the four ligands got substituted by two ligands.

Solubility

  • The resulting complex has a +2 charge ( and are uncharged + )
  • Since it is charged, it can easily form ion-dipole bonds with water molecules. It is therefore soluble.
  • Hence why the precipitate dissolves to form a dark blue solution.

Graph View