AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Co-ordination number is number of co-ordinate bonds to the central metal…
Full specification wording
General properties of transition metals (A-level only): Co-ordination number is number of co-ordinate bonds to the central metal atom or ion.
Co-ordination number counts co-ordinate bonds, not ligands, and the exam tests that distinction directly. [Cu(H₂O)₆]²⁺ has six monodentate water ligands and a co-ordination number of 6. [CuCl₄]²⁻…
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 5 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Co-ordination number counts co-ordinate bonds, not ligands, and the exam tests that distinction directly. [Cu(H₂O)₆]²⁺ has six monodentate water ligands and a co-ordination number of 6. [CuCl₄]²⁻ has four chloride ligands and a co-ordination number of 4. But [Cr(C₂O₄)₃]³⁻ has only three ethanedioate ligands, each bidentate, so its co-ordination number is still 6, and [Cu(EDTA)]²⁻ has a single multidentate ligand donating six pairs, so its co-ordination number is 6 as well. Small ligands such as H₂O, NH₃ and CN⁻ usually allow six around the metal, while bulky ligands such as Cl⁻ usually allow only four.
What earns the marks
- Defining co-ordination number as the number of co-ordinate bonds to the central metal atom or ion.
- Multiplying the number of ligands by the donor atoms each supplies when the ligand is bidentate or multidentate.
- The correct co-ordination number of a named complex, such as 6 in [Cr(C₂O₄)₃]³⁻.
- Linking a co-ordination number of 4 to larger ligands such as Cl⁻ and 6 to small ligands such as H₂O.
Common mistakes and how to put them right
Counting ligands instead of bonds, giving [Cr(C₂O₄)₃]³⁻ a co-ordination number of 3.
Giving [Cu(EDTA)]²⁻ a co-ordination number of 1 because it contains one ligand.
Confusing co-ordination number with the oxidation state of the metal.
Assuming every complex is six co-ordinate, so describing [CuCl₄]²⁻ as octahedral.
Exam tips
- Count donor atoms: the number of ligands multiplied by their denticity gives the co-ordination number.
- Quote the definition using the words co-ordinate bonds, because number of ligands is not accepted.
- Use the size of the ligand to predict 4 or 6 when a question gives you an unfamiliar complex.
Try the lesson
The start of the General properties of transition metals (A-level only): Co-ordination number is number of co-ordinate bonds to the central metal atom or ion. lesson, made from checked teaching notes for this specification point.
Have a go first
What is the co-ordination number of the central metal ion in the complex [Cr(C₂O₄)₃]³⁻?
Listen
Co-ordination number counts co-ordinate bonds, not ligands, and the exam tests that distinction directly. Cu(H₂O)₆]²⁺ has six monodentate water ligands and a co-ordination number of 6. CuCl₄]²⁻ has four chloride ligands and a co-ordination number of 4. But [Cr(C₂O₄)₃]³⁻ has only three ethanedioate ligands, each bidentate, so its co-ordination number is still 6, and [Cu(EDTA)]²⁻ has a single multidentate ligand donating six pairs, so its co-ordination number is 6 as well. Small ligands such as H₂O, NH₃ and CN⁻ usually allow six around the metal, while bulky ligands such as Cl⁻ usually allow only four. A strong answer earns marks for these points. defining co-ordination number as the number of co-ordinate bonds to the central metal atom or ion. multiplying the number of ligands by the donor atoms each supplies when the ligand is bidentate or multidentate. the correct co-ordination number of a named complex, such as 6 in [Cr(C₂O₄)₃]³⁻. linking a co-ordination number of 4 to larger ligands such as Cl⁻ and 6 to small ligands such as H₂O. Exam tip: Count donor atoms: the number of ligands multiplied by their denticity gives the co-ordination number.
Key words
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co-ordination number
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denticity
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six co-ordinate complexes
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four co-ordinate complexes
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See every definition
- co-ordination number
- The number of co-ordinate bonds to the central metal atom or ion.
- denticity
- The number of donor atoms each ligand supplies, so the number of ligands multiplied by their denticity gives the co-ordination number.
- six co-ordinate complexes
- Complexes with a co-ordination number of 6, usually allowed by small ligands such as H₂O, NH₃ and CN⁻.
- four co-ordinate complexes
- Complexes with a co-ordination number of 4, usually allowed by bulky ligands such as Cl⁻.
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