Ligand Substitution Reactions

Learning Objectives (OCR Spec 5.3.1)

By the end of this lesson you should be able to:

• Write balanced equations for ligand substitution in aqueous complexes
• Describe the colour changes when NH3 or Cl- replaces water in [Cu(H2O)6]2+
• Explain why coordination number may change during substitution (e.g. [Cu(H2O)6]2+ -> [CuCl4]2-)
• Recognise partial versus complete substitution and the role of ligand excess
• Write substitution reactions for [Co(H2O)6]2+ and [Cr(H2O)6]3+

What is a Ligand Substitution?

A ligand substitution is a reaction in which one ligand in a complex ion is replaced by another. The central metal ion is unchanged, its oxidation state is unchanged, and usually (but not always) the coordination number and shape are unchanged.

General form:

[M(L1)n]^a+ + x L2 -> [M(L1)(n-x)(L2)x]^a+ + x L1

Whether a new ligand substitutes depends on how strongly it binds compared with the original ligand. Generally, anions and neutral species that donate strongly and fit well into the octahedral coordination sphere win out. OCR asks you to remember specific reactions and colour changes for [Cu(H2O)6]2+ (the most important), [Co(H2O)6]2+, [Cr(H2O)6]3+, and a few others.

The [Cu(H2O)6]2+ Substitution Series

This is the single most examined reaction set in OCR A-Level transition metal chemistry. Learn all four steps.

Step 1: Add Dilute NH3 (or NaOH) - Deprotonation

Adding a small amount of ammonia solution to [Cu(H2O)6]2+(aq) produces a pale blue precipitate of copper(II) hydroxide:

[Cu(H2O)6]2+(aq) + 2 NH3(aq) -> Cu(OH)2(H2O)4(s) + 2 NH4+(aq)

Or, more precisely:

[Cu(H2O)6]2+ + 2 OH- -> Cu(OH)2(H2O)4 + 2 H2O

This is technically an acid-base reaction (not a substitution), with NH3 removing protons from coordinated water. OCR tests it alongside ligand substitution.

Observation: pale blue solution -> pale blue precipitate (gelatinous).

Step 2: Add Excess NH3 - Ligand Substitution

If you then add excess ammonia, the precipitate dissolves to give a deep royal blue solution:

Cu(OH)2(H2O)4(s) + 4 NH3(aq) -> [Cu(NH3)4(H2O)2]2+(aq) + 2 H2O(l) + 2 OH-(aq)

Or overall from the original complex:

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

Notice that only four waters are replaced, not all six. The four equatorial (in-plane) waters of the octahedron are substituted preferentially; the two axial waters remain coordinated. This is due to the Jahn-Teller distortion of d9 Cu2+ ions, which makes axial bonds longer and weaker and the equatorial sites more reactive.

Observation: pale blue precipitate dissolves -> deep royal blue solution.

Step 3: Add Concentrated HCl - Substitution with Change of Coordination Number

Adding concentrated HCl to [Cu(H2O)6]2+ causes all six waters to be replaced by chloride ions, but only four chlorides fit around the copper because chloride is larger than water. The new complex is tetrahedral with coordination number 4:

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

Observation: pale blue -> yellow (at high [Cl-]). An intermediate green colour is often seen because the solution contains a mixture of blue [Cu(H2O)6]2+ and yellow [CuCl4]2-.

Note:

• Coordination number changes from 6 to 4
• Shape changes from octahedral to tetrahedral
• Charge of the complex changes from +2 to -2
• The reaction is reversible - dilution or addition of water will shift the equilibrium back to [Cu(H2O)6]2+.

Step 4: Alternative: Add NaOH - Hydroxide Precipitation

Adding NaOH(aq) gives the same pale blue precipitate as ammonia:

[Cu(H2O)6]2+(aq) + 2 OH-(aq) -> Cu(OH)2(H2O)4(s) + 2 H2O(l)

Unlike ammonia, NaOH does NOT dissolve the precipitate in excess (copper hydroxide is not amphoteric). The pale blue precipitate persists.

Summary Table for Cu2+

Reagent	Product	Colour	Coord. no.	Shape
Start	[Cu(H2O)6]2+	Pale blue	6	Octahedral
Add OH- (or few drops NH3)	Cu(OH)2(H2O)4	Pale blue ppt	6	Octahedral
Add excess NH3	[Cu(NH3)4(H2O)2]2+	Deep royal blue	6	Octahedral
Add conc. HCl	[CuCl4]2-	Yellow	4	Tetrahedral

Similar Substitutions for Co2+

Starting from the pink aqua complex: