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5.2.2Unit 5. Transition Metals and Organic Nitrogen Chemistry

Transition Metal Complexes — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 5 parts Lesson 14 of 43 Unit quiz

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Part 1 of 5Transition Metals

5.2.2 Transition Metal Complexes
Definition
  • A transition metal forms at least one ion with a partially filled d subshell.
Exceptions
  • Scandium (Sc³⁺, 3d⁰) and zinc (Zn²⁺, 3d¹⁰) are d-block elements but do not form such ions, so they are not transition metals.
Electron configurations
  • Iron is [Ar]3d⁶4s², and Fe²⁺ is [Ar]3d⁶ because the 4s electrons are lost first when forming ions.
Anomalies
  • Chromium is [Ar]3d⁵4s¹ and copper is [Ar]3d¹⁰4s¹, because half-filled and filled d subshells are especially stable.

Part 2 of 5Characteristic Properties

Characteristic Properties
Variable oxidation states
  • Similar energies of the 3d and 4s electrons allow several different numbers of electrons to be lost.
Coloured compounds
  • Partially filled d orbitals allow absorption of visible light, as covered in a later lesson.
Catalysis
  • Many transition metals and their compounds provide surfaces or alternative reaction pathways.
Complex formation
  • Small, highly charged ions with empty orbitals accept lone pairs from ligands.

Part 3 of 5Complex Ions

1
A complex consists of a central metal ion surrounded by ligands that donate lone pairs to form dative covalent bonds.
2
The coordination number is the number of dative bonds to the central ion, commonly 6 or 4.
3
The overall charge is the sum of the charge on the central ion and the charges on the ligands, for example [Fe(CN)₆]³⁻.
4
Square brackets enclose the complex, with the metal first and ligands after it.
5
The oxidation number of the metal is found by subtracting the total ligand charge from the overall charge.

Part 4 of 5Worked Example

Question Find the oxidation number of iron in [Fe(CN)₆]³⁻.

  1. Each cyanide ligand has a charge of -1, so six ligands contribute -6.
  2. Overall charge = oxidation number of Fe + (-6) = -3.
  3. Oxidation number of Fe = -3 + 6 = +3.

Answer Fe has oxidation number +3

Part 5 of 5Exam Checklist & Tips

5.2.2 – Key Points for the Exam

  • Transition metal An element that forms at least one ion with an incomplete d subshell.
  • Ligand A molecule or ion that donates a lone pair to a metal ion.
  • Coordination number The number of dative bonds around the central ion.
  • Complex ion A central metal ion bonded to ligands by dative covalent bonds.

Common Mistakes

  • Including Sc and Zn as transition metals.
  • Removing 3d electrons before 4s electrons when forming ions.
  • Forgetting to include ligand charges when finding the oxidation number.
  • Confusing coordination number with the number of ligands for bidentate ligands.

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