5.2.4Unit 5. Transition Metals and Organic Nitrogen Chemistry
Ligands — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions
5 min read 5 parts Lesson 16 of 43 Unit quiz
Part 1 of 5Types of Ligand
Monodentate
- These donate one lone pair, as with H₂O, NH₃, Cl⁻, CN⁻ and OH⁻.
Bidentate
- These donate two lone pairs from two atoms, as with 1,2-diaminoethane (NH₂CH₂CH₂NH₂, 'en') and the ethanedioate ion (C₂O₄²⁻).
Polydentate
- Ligands with more than two donor atoms;
- EDTA⁴⁻ is hexadentate and wraps round the metal ion using six donor atoms.
Dative bonding
- Each donor atom uses a lone pair to form a coordinate bond, and both bonding electrons come from the ligand.
Part 2 of 5Coordination Number Counting
Coordination Number Counting
Aqua complex
- [Cu(H₂O)₆]²⁺ has six monodentate ligands, so the coordination number is 6.
With bidentate
- [Ni(en)₃]²⁺ contains three bidentate ligands, so the coordination number is 6 even though only three ligands are present.
With EDTA
- Each EDTA⁴⁻ binds through six atoms, so a single EDTA ion fills all six positions around a metal ion.
Part 3 of 5The Chelate Effect
1
Replacing monodentate ligands with bidentate or polydentate ligands often gives a more stable complex.
2
For [Ni(H₂O)₆]²⁺ + 3en → [Ni(en)₃]²⁺ + 6H₂O, four particles on the left become seven on the right.
3
The number of independent particles increases, so the entropy of the system rises and ΔS is positive.
4
The enthalpy change is small because six Ni-N or Ni-O type dative bonds are replaced by six similar ones.
5
With ΔH close to zero and ΔS positive, ΔG = ΔH - TΔS is negative and the substitution is favourable.
Part 4 of 5Worked Example
Question Find the coordination number and the oxidation number of chromium in [Cr(C₂O₄)₃]³⁻.
- Each ethanedioate ion is bidentate with a charge of -2, and there are three of them.
- Coordination number = 3 x 2 = 6.
- Oxidation number of Cr + (3 x -2) = -3, so Cr = -3 + 6 = +3.
Answer
- Coordination number 6;
- oxidation number +3
Part 5 of 5Exam Checklist & Tips
5.2.4 – Key Points for the Exam
- Monodentate A ligand that forms one dative bond to the metal.
- Bidentate A ligand that forms two dative bonds to the metal.
- Chelate effect Extra stability of complexes with polydentate ligands, due to entropy.
- EDTA A hexadentate ligand used to bind metal ions.
Common Mistakes
- Counting a bidentate ligand as one dative bond.
- Saying the chelate effect is due to stronger bonds; it is mainly entropy.
- Drawing ligand arrows from the metal rather than from the ligand lone pair.
- Forgetting the charge on EDTA⁴⁻ when working out complex charge.
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