Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

5.2.6Unit 5. Transition Metals and Organic Nitrogen Chemistry

Octahedral and Tetrahedral Complexes — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 18 of 43 Unit quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Part 1 of 5Shapes

5.2.6 Octahedral and Tetrahedral Complexes
Octahedral
  • Six ligands surround the metal with bond angles of 90°, typical for small ligands such as H₂O, NH₃ and CN⁻.
Tetrahedral
  • Four ligands surround the metal with bond angles of 109.5°, typical for larger ligands such as Cl⁻.
Reason for four
  • Large chloride ions repel each other too strongly for six to fit around the metal ion.
Examples
  • [Cu(H₂O)₆]²⁺ and [Fe(CN)₆]³⁻ are octahedral, while [CuCl₄]²⁻ and [CoCl₄]²⁻ are tetrahedral.
Naming
  • Ligand names in a complex precede the metal, so [Co(NH₃)₆]³⁺ is hexaamminecobalt(III), showing both ligand number and oxidation state.

Part 2 of 5Cis-Trans Isomerism

Cis-Trans Isomerism
Octahedral cis-trans
  • In [Co(NH₃)₄Cl₂]⁺ the two chloride ligands can be next to each other (cis, 90°) or opposite each other (trans, 180°).
Why
  • The ligands occupy different positions in space but have the same bonds, making them stereoisomers.
Properties
  • The isomers may differ in colour or in polarity, since the trans form is symmetric.
Detection
  • Cis and trans forms can sometimes be told apart by their different dipole moments, since the trans isomer often has no overall dipole.

Part 3 of 5Optical Isomerism

1
Octahedral complexes with three bidentate ligands, such as [Ni(en)₃]²⁺, have a three-bladed propeller shape.
2
The structure and its mirror image cannot be superimposed, so the two forms are non-superimposable mirror images.
3
These enantiomers rotate plane polarised light in opposite directions.
4
A cis form of [Co(en)₂Cl₂]⁺ is also optically active, whereas the trans form has a plane of symmetry and is not.
5
Optical isomerism in complexes therefore needs a lack of symmetry rather than a carbon atom with four different groups.

Part 4 of 5Worked Example

Question State the coordination number and shape of [CuCl₄]²⁻ and the oxidation number of copper.

  1. Four chloride ligands are attached, so coordination number = 4 and the shape is tetrahedral.
  2. Four Cl⁻ contribute -4, and the overall charge is -2.
  3. Oxidation number of Cu = -2 + 4 = +2.

Answer Coordination number 4, tetrahedral, copper +2

Part 5 of 5Exam Checklist & Tips

5.2.6 – Key Points for the Exam

  • Octahedral Six-coordinate geometry with 90° bond angles.
  • Tetrahedral Four-coordinate geometry with 109.5° bond angles.
  • Cis-trans isomers Isomers differing in the relative positions of identical ligands.
  • Enantiomers Non-superimposable mirror image isomers.

Common Mistakes

  • Stating that all four-coordinate complexes are tetrahedral.
  • Drawing bond angles in octahedral complexes as 109.5°.
  • Describing cis isomers as those with ligands on opposite sides.
  • Saying all octahedral complexes are optically active.

Last updated:

\n