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Chemistry · 10. Coordination chemistry and complex ions

Coordination compounds, coordinate bonding and crystal field theory

Investigate central metal atoms, ligands, coordination numbers, geometries and biomedical applications of complexes.

40 minutesContent version: 2.1
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Driving question

Why is Cu²⁺ colourless as a free gas ion but turns sky-blue in water and deep royal-blue upon adding ammonia?

Curriculum coverage

Vietnam 2018 Chemistry 12 (Coordination compounds); Cambridge A-Level Chemistry 9701 (Transition elements).

Model scope and limitations

The model applies to aqueous solutions under standard conditions, focusing on first-row transition metal complexes.

Learning objectives

  • Identify central ions, ligands, complex sphere charges and coordination numbers.
  • Explain colour changes via d-orbital splitting (t2g and eg) and ligand exchange reactions.
  • Distinguish square planar, tetrahedral and octahedral geometries, and Cisplatin’s anticancer action.

Prerequisite knowledge

  • Coordinate (dative covalent) bonding and lone pairs.
  • Transition metal 3d subshell electron configurations.

Core knowledge

Complex structure

Consists of a central atom/ion bonded to ligands via coordinate bonds (ligand lone pairs donate into vacant metal orbitals).

Ligand exchange and Chelate effect

Polydentate ligands (like EDTA, en) form far more stable chelates than monodentate ligands due to positive entropy change (ΔS > 0).

Colour origin and d-d splitting

Ligand fields split degenerate d-orbitals; electrons absorb visible photons to transition between levels, transmitting the complementary colour.

Worked example

In [Cu(NH₃)₄(H₂O)₂]²⁺, Cu²⁺ is the central ion with coordination number 6. Adding EDTA⁴⁻ displaces all monodentate ligands because of an immense stability constant (log K ≈ 18.8), yielding the stable chelate [Cu(EDTA)]²⁻.

Misconceptions to avoid

  • Coordination number is not equal to central ion charge; it reflects the total number of σ-bonds formed with ligands.
  • Sc³⁺ (3d⁰) and Zn²⁺ (3d¹⁰) are colourless because d-d electronic transitions cannot occur in empty or fully filled d shells.

Virtual experiment procedure: EDTA complexometric titration and complex ions

  1. Prepare dilute Cu²⁺ or Fe³⁺ solution in a test tube or conical flask.
  2. Add ligand solution (NH₃ or KSCN) dropwise and observe abrupt spectral colour transitions.
  3. Add excess strong chelating agent (such as Na₂H₂EDTA) to observe competitive equilibrium displacement.

Safety and cautions

  • Concentrated ammonia emits noxious fumes; operate in a fume hood and wear safety goggles.

Evidence to collect

  • Table recording ligand concentrations, solution colours and peak absorption wavelengths.
  • Net ionic equation for ligand displacement with the stability constant expression K_stab.
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<iframe src="https://visuallab.win/chem-lab/?experiment=exp_univ_edta_water_hardness" width="100%" height="560" frameborder="0" allow="fullscreen" loading="lazy"></iframe>

Diagnostic check

1. In the octahedral complex [Fe(CN)₆]³⁻, what is the coordination number of Fe³⁺?

  1. 6
  2. 3
  3. −3
Show answer and explanation

Correct answer: 6

Six monodentate CN⁻ ligands coordinate to the centre, so the coordination number is 6.

2. Why is Cisplatin [Pt(NH₃)₂Cl₂] an active anticancer drug while Transplatin is ineffective?

  1. Cisplatin has adjacent Cl ligands that form intrastrand cross-links on DNA
  2. Transplatin cannot dissolve in water
  3. Transplatin has a larger molar mass
Show answer and explanation

Correct answer: Cisplatin has adjacent Cl ligands that form intrastrand cross-links on DNA

Adjacent cis-Cl ligands (90° angle) match the spatial distance required to cross-link adjacent guanine bases on DNA.

Application practice

1. What is the charge of the complex ion formed by Co³⁺ and 6 NH₃ ligands?

  1. +3
  2. 0
  3. +6
Show answer and explanation

Correct answer: +3

NH₃ is a neutral ligand (charge 0), so complex charge equals Co³⁺ charge (+3).

2. Which of the following is a bidentate ligand?

  1. 1,2-diaminoethane (en)
  2. Chloride ion (Cl⁻)
  3. Water molecule (H₂O)
Show answer and explanation

Correct answer: 1,2-diaminoethane (en)

1,2-diaminoethane possesses two nitrogen atoms each with a lone pair capable of simultaneous coordination.

3. Why are Zn²⁺ complexes ([Zn(H₂O)₆]²⁺) colourless in aqueous solution?

  1. Filled d¹⁰ configuration prevents d-d transitions
  2. Ionic radius is too large
  3. It forms no coordinate bonds
Show answer and explanation

Correct answer: Filled d¹⁰ configuration prevents d-d transitions

Zn²⁺ has a closed-shell [Ar]3d¹⁰ configuration; with fully occupied d-orbitals, no d-d transitions can occur.

Academic references

  1. Cambridge IGCSE Chemistry 0620 — official syllabus and resources
  2. Cambridge AS & A Level Chemistry 9701 — official syllabus
  3. OpenStax Chemistry 2e — peer-reviewed general chemistry
  4. IUPAC Color Books — authoritative chemical terminology and nomenclature
  5. BIPM — SI Brochure, 9th edition

This lesson is maintained against the cited sources below. Simulations are learning models and do not replace supervised physical-laboratory safety procedures.

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