Classify ligands and define complex ions
Count ligand donor atoms, name ligand denticity and use coordination number to describe a metal complex.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 28. Use the first-row transition elements titanium to copper and the species explicitly named in the syllabus. Where redox potentials or stability constants are needed, use the values supplied in the question. Colour descriptions can depend on conditions and concentration. Written practice is self-checked
Before you begin
- Use electron configurations, oxidation numbers, ionic equations, equilibrium expressions and the AS redox and bonding models.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which named ligand is bidentate?
What you will learn
- Define ligand and complex using a central metal species and donated lone pairs
- Classify H₂O, NH₃, Cl⁻ and CN⁻ as monodentate, en and C₂O₄²⁻ as bidentate, and EDTA⁴⁻ as polydentate
- Define coordination number by counting donor atoms bonded to the central metal
Ligands donate lone pairs to a central metal
A ligand is a species containing a lone pair that forms a dative covalent bond to a central metal atom or ion. A complex is a molecule or ion in which a central metal atom or ion is surrounded by one or more ligands.
A ligand can bind through one or several donor atoms. Count the atoms that actually donate lone pairs to the same central metal when finding the coordination number.
Use denticity to describe the number of donor atoms
H₂O, NH₃, Cl⁻ and CN⁻ are monodentate examples: each attaches through one donor atom. The bidentate ligands named in the syllabus are 1,2-diaminoethane, en, and ethanedioate, C₂O₄²⁻. Each can bind through two donor atoms.
EDTA⁴⁻ is the named polydentate ligand. Polydentate means that more than two donor atoms from one ligand can bind to the same metal. Denticity is a property of the ligand; coordination number counts all metal–donor-atom bonds around the central metal.
Pause and explain
What does coordination number count?
Worked example
In [Ni(en)₃]²⁺, identify the ligand denticity and the coordination number.
Show the worked solution
- Each en ligand binds through two nitrogen donor atoms.
- There are three en ligands.
- Count all Ni–N coordinate bonds.
Answer en is bidentate. Three en ligands provide six donor atoms, so the coordination number of Ni is 6.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Classify H₂O, C₂O₄²⁻ and EDTA⁴⁻ by denticity using the syllabus examples.
Give me a hint
Use the number of donor atoms by which each ligand can attach.
Compare my reasoning
- H₂O has one donor atom in this classification.
- Ethanedioate has two donor atoms.
- EDTA⁴⁻ is the named polydentate ligand.
H₂O is monodentate; C₂O₄²⁻ is bidentate; EDTA⁴⁻ is polydentate.
Look for these in your work
- H₂O classification correct
- Ethanedioate classification correct
- EDTA classification correct
Find the coordination number in [Co(en)₂(H₂O)₂]²⁺.
Give me a hint
Each en contributes two donor atoms; each water contributes one.
Compare my reasoning
- Two en ligands contribute 2 × 2 = 4 donors.
- Two water ligands contribute 2 × 1 = 2 donors.
- Add all donors attached to Co.
Coordination number = 4 + 2 = 6.
Look for these in your work
- Denticity of en used
- Water donor count used
- Total calculated
A learner says [Fe(CN)₆]⁴⁻ has coordination number 1 because it contains one formula unit of the ligand CN⁻. Correct the statement.
Give me a hint
The coordination number counts individual metal–donor bonds, not ligand identities.
Compare my reasoning
- CN⁻ is monodentate.
- Six CN⁻ ligands are attached.
- Each contributes one donor atom to Fe.
The coordination number is 6: six monodentate CN⁻ ligands each donate one lone pair to Fe. The overall charge −4 is not the coordination number.
Look for these in your work
- Ligand denticity identified
- Six donor atoms counted
- Charge distinguished
Common mistakes
- Counting ligands instead of donor atoms for coordination number
- Calling en monodentate because it is one molecule
- Including the central metal in the ligand count
- Confusing the overall ion charge with coordination number
What can you explain now?
Which named ligand is bidentate?
Compare with the explanation
1,2-diaminoethane (en)
It has two nitrogen donor atoms that can bind to one metal.
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Make it stick
How do ligand denticity and coordination number differ, and which atom in each ligand supplies the donor pair?
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