AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Transition metal ions commonly form tetrahedral – complexes with larger…
Full specification wording
Shapes of complex ions (A-level only): Transition metal ions commonly form tetrahedral – complexes with larger ligands (eg Cl ).
Ligand size decides the shape.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Ligand size decides the shape. Small ligands such as H₂O, NH₃ and CN⁻ pack six around a metal ion to give an octahedral complex with bond angles of 90°. Chloride is much bigger, so only four fit, and those four arrange themselves tetrahedrally with bond angles of 109.5°. [CuCl₄]²⁻ and [CoCl₄]²⁻ are the standard examples and both are tetrahedral, not square planar, a confusion that a multiple choice paper has tested directly. Adding concentrated hydrochloric acid to aqueous copper(II) shows the swap: pale blue octahedral [Cu(H₂O)₆]²⁺ becomes yellow tetrahedral [CuCl₄]²⁻, the co-ordination number falling from 6 to 4 and the colour changing as a result.
What earns the marks
- Identifying a chloride complex such as [CoCl₄]²⁻ or [CuCl₄]²⁻ as tetrahedral.
- The bond angle, 109.5° in a tetrahedral complex against 90° in an octahedral one.
- Attributing the co-ordination number of 4 to the larger size of the chloride ligand.
- A correct substitution equation that includes all six displaced water molecules.
Common mistakes and how to put them right
Calling [CoCl₄]²⁻ square planar, which is the distractor used in the multiple choice papers.
Quoting 90° as the bond angle in a tetrahedral chloride complex.
Keeping six ligands when writing the chloride complex, so giving [CuCl₆]⁴⁻.
Explaining the change of shape by the charge on chloride rather than by its size.
Exam tips
- Read the ligand first, because Cl⁻ almost always means four ligands and a tetrahedral shape.
- Give the bond angle whenever you name a shape, since it is often a separate mark.
- Show all six water molecules on the right hand side when chloride replaces them.
Try the lesson
The start of the Shapes of complex ions (A-level only): Transition metal ions commonly form tetrahedral – complexes with larger ligands (eg Cl ). lesson, made from checked teaching notes for this specification point.
Have a go first
Which shape is commonly adopted by complex ions, such as [CoCl₄]²⁻ and [CuCl₄]²⁻, where a transition metal ion bonds to four large chloride ligands?
Listen
Ligand size decides the shape. Small ligands such as H₂O, NH₃ and CN⁻ pack six around a metal ion to give an octahedral complex with bond angles of 90°. Chloride is much bigger, so only four fit, and those four arrange themselves tetrahedrally with bond angles of 109.5°. CuCl₄]²⁻ and [CoCl₄]²⁻ are the standard examples and both are tetrahedral, not square planar, a confusion that a multiple choice paper has tested directly. Adding concentrated hydrochloric acid to aqueous copper(II) shows the swap: pale blue octahedral [Cu(H₂O)₆]²⁺ becomes yellow tetrahedral [CuCl₄]²⁻, the co-ordination number falling from 6 to 4 and the colour changing as a result. A strong answer earns marks for these points. identifying a chloride complex such as [CoCl₄]²⁻ or [CuCl₄]²⁻ as tetrahedral. the bond angle, 109.5° in a tetrahedral complex against 90° in an octahedral one. attributing the co-ordination number of 4 to the larger size of the chloride ligand. a correct substitution equation that includes all six displaced water molecules. Exam tip: Read the ligand first, because Cl⁻ almost always means four ligands and a tetrahedral shape.
See it
![A top-down flowchart. On the left, a node labelled 'Small ligands: H2O, NH3, CN-' leads to 'Six ligands pack around metal ion', which leads to 'Octahedral complex', which leads to 'Bond angles 90 degrees'. Separately, a node labelled 'Larger ligand: Cl-' leads to 'Only four ligands fit', which leads to 'Tetrahedral complex', which leads to 'Bond angles 109.5 degrees'. From the 'Octahedral complex' node, an arrow leads to 'Pale blue [Cu(H2O)6]2+', which leads to 'Add concentrated hydrochloric acid', which leads to 'Yellow [CuCl4]2-', and this final node leads into the 'Tetrahedral complex' node.](https://xnnrgnazirrqvdgfhvou.supabase.co/storage/v1/object/public/lesson-media/e85cfcab-23a3-4345-b1b5-5799742a54b9/diagram-b27cadcf06d5.svg)
Key words
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tetrahedral complexes
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ligand size
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co-ordination number 4
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bond angle of 109.5°
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See every definition
- tetrahedral complexes
- Complexes in which four ligands arrange themselves tetrahedrally around the metal ion, with bond angles of 109.5°.
- ligand size
- The factor that decides the shape, because small ligands pack six around a metal ion while larger ligands allow only four to fit.
- co-ordination number 4
- The number of ligands attached to the metal ion when larger ligands such as chloride are present, falling from 6.
- bond angle of 109.5°
- The angle found in a tetrahedral complex, against 90° in an octahedral one.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
![The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. On the left, a node labelled 'Small ligands: H2O, NH3, CN-' leads to 'Six ligands pack around metal ion', which leads to 'Octahedral complex', which leads to 'Bond angles 90 degrees'. Separately, a node labelled 'Larger ligand: Cl-' leads to 'Only four ligands fit', which leads to 'Tetrahedral complex', which leads to 'Bond angles 109.5 degrees'. From the 'Octahedral complex' node, an arrow leads to 'Pale blue [Cu(H2O)6]2+', which leads to 'Add concentrated hydrochloric acid', which leads to 'Yellow [CuCl4]2-', and this final node leads into the 'Tetrahedral complex' node.](https://xnnrgnazirrqvdgfhvou.supabase.co/storage/v1/object/public/lesson-media/e85cfcab-23a3-4345-b1b5-5799742a54b9/diagram-labelled-b27cadcf06d5.svg)
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