Why does exchange of NH₃ and H₂O not change the co-ordination number?

They are similar in size and both are uncharged.

Give the equation for [Cu(H₂O)₆]²⁺ reacting with excess ammonia.

[Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O

What does incomplete substitution mean?

Only some of the ligands are replaced, as in [Cu(NH₃)₄(H₂O)₂]²⁺.

Why can exchange of H₂O by Cl⁻ change the co-ordination number?

Cl⁻ is larger than H₂O and NH₃, so fewer ligands fit around the central ion.

Give the equation for [Co(H₂O)₆]²⁺ reacting with chloride ions.

[Co(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CoCl₄]²⁻ + 6H₂O

What is a bidentate ligand?

A ligand that forms two co-ordinate bonds to the central metal ion.

Name two bidentate ligands.

Ethane-1,2-diamine, H₂NCH₂CH₂NH₂, and the ethanedioate ion, C₂O₄²⁻.

What is a multidentate ligand? Give an example.

A ligand forming more than two co-ordinate bonds; EDTA⁴⁻ forms six.

An iron(II) complex with a multidentate ligand, found in haemoglobin.

How does haemoglobin carry oxygen?

Oxygen forms a reversible co-ordinate bond to the Fe(II) in haem.

Why is carbon monoxide toxic?

It replaces oxygen co-ordinately bonded to Fe(II) in haemoglobin, forming a stronger bond, so oxygen can no longer be carried.

What is the chelate effect?

Bidentate and multidentate ligands replace monodentate ligands to form more stable complexes.

Explain the chelate effect using entropy and enthalpy.

The number of particles increases, so entropy increases; ΔH is about zero as the same number and type of bonds are made; so ΔG becomes negative.

Ligand substitution and the chelate effectAQA A-Level Chemistry: Flashcards

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What is ligand substitution?

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What is ligand substitution?: A reaction in which one or more ligands in a complex are replaced by other ligands.
Why does exchange of NH₃ and H₂O not change the co-ordination number?: They are similar in size and both are uncharged.
Give the equation for [Cu(H₂O)₆]²⁺ reacting with excess ammonia.: [Cu(H₂O)₆]²⁺ + 4NH₃ ⇌ [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O
What does incomplete substitution mean?: Only some of the ligands are replaced, as in [Cu(NH₃)₄(H₂O)₂]²⁺.
Why can exchange of H₂O by Cl⁻ change the co-ordination number?: Cl⁻ is larger than H₂O and NH₃, so fewer ligands fit around the central ion.
Give the equation for [Co(H₂O)₆]²⁺ reacting with chloride ions.: [Co(H₂O)₆]²⁺ + 4Cl⁻ ⇌ [CoCl₄]²⁻ + 6H₂O
What is a bidentate ligand?: A ligand that forms two co-ordinate bonds to the central metal ion.
Name two bidentate ligands.: Ethane-1,2-diamine, H₂NCH₂CH₂NH₂, and the ethanedioate ion, C₂O₄²⁻.
What is a multidentate ligand? Give an example.: A ligand forming more than two co-ordinate bonds; EDTA⁴⁻ forms six.
What is haem?: An iron(II) complex with a multidentate ligand, found in haemoglobin.
How does haemoglobin carry oxygen?: Oxygen forms a reversible co-ordinate bond to the Fe(II) in haem.
Why is carbon monoxide toxic?: It replaces oxygen co-ordinately bonded to Fe(II) in haemoglobin, forming a stronger bond, so oxygen can no longer be carried.
What is the chelate effect?: Bidentate and multidentate ligands replace monodentate ligands to form more stable complexes.
Explain the chelate effect using entropy and enthalpy.: The number of particles increases, so entropy increases; ΔH is about zero as the same number and type of bonds are made; so ΔG becomes negative.

Exam questions on Ligand substitution and the chelate effect

Aqueous copper(II) sulfate contains the pale blue complex ion [Cu(H₂O)₆]²⁺. When an excess of concentrated ammonia solution is added the colour changes to deep blue, but the product is [Cu(NH₃)₄(H₂O)₂]²⁺ rather than [Cu(NH₃)₆]²⁺.
Explain why the co-ordination number of copper does not change when ammonia replaces water in this reaction.2 marks
A student investigates reactions of aqueous metal ions with concentrated hydrochloric acid. Aqueous copper(II) sulfate contains [Cu(H₂O)₆]²⁺ and aqueous iron(III) nitrate contains [Fe(H₂O)₆]³⁺. Each reacts with the acid to form a yellow-green solution of [CuCl₄]²⁻ and a yellow solution of [FeCl₄]⁻ respectively.
Write an equation for the formation of [CuCl₄]²⁻ from [Cu(H₂O)₆]²⁺ and state the change in co-ordination number of copper.2 marks
Nickel(II) ions in aqueous solution form [Ni(H₂O)₆]²⁺, and adding excess ammonia solution gives [Ni(NH₃)₆]²⁺. When ethane-1,2-diamine, H₂NCH₂CH₂NH₂ (en), is added, the ammonia ligands are replaced: [Ni(NH₃)₆]²⁺ + 3en → [Ni(en)₃]²⁺ + 6NH₃. This reaction goes essentially to completion at room temperature.
Explain why en is described as a bidentate ligand and state the co-ordination number of nickel in [Ni(en)₃]²⁺.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).