6.4.6Additional lessons
Coordination Bonds — IB Chemistry HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 153 of 172
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Complex Ion Structure
A complex ion forms when ligands bond to a central metal ion through lone pairs.
Central ion
- Central ion A complex ion has a metal ion, usually a d-block ion, at its centre acting as a Lewis acid.
Ligands
- Ligands Ligands such as H₂O, NH₃, Cl⁻ and CN⁻ each donate a lone pair to the metal ion.
Coordinate bond
- Coordinate bond Each metal-ligand bond is a coordinate bond, in which both bonding electrons come from the ligand.
Coordination number
- Coordination number This is the number of coordinate bonds to the central ion, usually 2, 4 or 6.
Section 2 of 5Writing and Reading Formulas
1
Write the central metal ion first, then the ligands, and place the whole complex inside square brackets.
2
The charge on the complex equals the charge on the metal ion plus the charges of all ligands.
3
Neutral ligands such as H₂O and NH₃ add no charge, whereas Cl⁻ and CN⁻ each add −1.
4
For [Fe(CN)₆]³⁻ the six cyanide ligands total −6, so iron must be +3 to give an overall charge of −3.
5
For [CuCl₄]²⁻ the four chloride ligands total −4, so the copper is +2.
| Complex ion | Ligand | Coordination number | Metal oxidation state |
|---|---|---|---|
| [Cu(NH₃)₄]²⁺ | NH₃ | 4 | +2 |
| [Fe(H₂O)₆]³⁺ | H₂O | 6 | +3 |
| [CoCl₄]²⁻ | Cl⁻ | 4 | +2 |
| [Fe(CN)₆]³⁻ | CN⁻ | 6 | +3 |
Section 3 of 5Types of Ligand
Types of Ligand
Monodentate
- Monodentate Each monodentate ligand, such as H₂O or Cl⁻, forms one coordinate bond using one lone pair.
Bidentate
- Bidentate A bidentate ligand such as ethane-1,2-diamine forms two coordinate bonds through two donor atoms.
Polydentate
- Polydentate Larger ligands like EDTA⁴⁻ have several donor atoms and wrap around the metal ion, forming very stable complexes.
Colour
- Colour Many complexes are coloured, and changing the ligand often changes the colour, as when Cu²⁺ turns deep blue with ammonia.
Section 4 of 5Worked Example
Question Determine the oxidation state of cobalt and the coordination number in [Co(NH₃)₄Cl₂]⁺.
- NH₃ is neutral and each Cl⁻ contributes −1, so the ligands contribute −2.
- Let the oxidation state of Co be x: x + (−2) = +1, so x = +3.
- There are four NH₃ and two Cl⁻ ligands, each monodentate, so six coordinate bonds are formed.
Answer Cobalt is +3, and the coordination number is 6.
Section 5 of 5Exam Checklist & Tips
6.4.6 – Key Points for the Exam
- Ligand A species that donates a lone pair of electrons to a central metal ion.
- Coordination number The number of coordinate bonds formed to the central ion.
- Complex ion A central metal ion surrounded by ligands bonded by coordinate bonds.
- Monodentate A ligand that forms only one coordinate bond to the metal ion.
Common Mistakes
- Counting the number of ligands as the coordination number for bidentate ligands, which give two bonds each.
- Forgetting that the ligand charges contribute to the overall charge.
- Writing the charge of the complex inside the square brackets.
- Calling the metal ion the Lewis base, when it is the acceptor.
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