Complex ions, ligands and shapesEdexcel International A Level Chemistry: Subtopic test

1. 1
   When an excess of aqueous ammonia is added to aqueous copper(II) sulfate, some of the water ligands around the copper(II) ion are replaced and the complex ion [Cu(NH₃)₄(H₂O)₂]²⁺ forms.

   (a)

   What is the coordination number of copper in [Cu(NH₃)₄(H₂O)₂]²⁺?

   [1 mark]

   • A6
   • B4
   • C2
   • D8

   (b)

   Which statement correctly describes the bond between an NH₃ ligand and the copper(II) ion?

   [1 mark]

   • AAn ionic attraction between oppositely charged ions
   • BA covalent bond in which each atom provides one electron
   • CA metallic bond with delocalised electrons
   • DA dative covalent bond in which both electrons come from the nitrogen atom

   (c)

   Explain why ammonia is able to act as a ligand.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Cobalt(II) ions form the complex ion [Co(H₂O)₆]²⁺ in water. When concentrated hydrochloric acid is added, the complex ion [CoCl₄]²⁻ forms.

   (a)

   What is the shape of the [Co(H₂O)₆]²⁺ ion?

   [1 mark]

   • ATetrahedral
   • BSquare planar
   • COctahedral
   • DTrigonal bipyramidal

   (b)

   What is the shape of the [CoCl₄]²⁻ ion?

   [1 mark]

   • AOctahedral
   • BTetrahedral
   • CSquare planar
   • DLinear

   (c)

   Suggest why the cobalt(II) ion bonds to four chloride ligands but to six water ligands.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Cisplatin, [Pt(NH₃)₂Cl₂], is a neutral, square planar complex of platinum used in cancer treatment. It is supplied to hospitals as a single isomer.

   (a)

   State the coordination number of platinum in cisplatin and deduce the oxidation number of platinum in it. State the shape of the complex.

   [3 marks]

   (b)

   Describe the difference between the cis and trans isomers of this complex and explain why cisplatin is supplied as a single isomer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Complex ions matter in biology and medicine. Haemoglobin is an iron(II) complex that carries oxygen around the body by bonding to O₂ molecules as ligands. Carbon monoxide is poisonous, and the hexadentate ligand EDTA⁴⁻ is used to remove toxic metal ions such as Pb²⁺ from the body.

   (a)

   Explain why carbon monoxide is poisonous, in terms of the bonding in haemoglobin.

   [6 marks]

   (b)

   Explain what is meant by hexadentate and deduce how many ligands are needed to give a coordination number of 6 with EDTA⁴⁻ and with the bidentate ligand ethane-1,2-diamine, NH₂CH₂CH₂NH₂. Write an equation for the complex EDTA⁴⁻ forms with Pb²⁺.

   [6 marks]

   Total for question 4: 12 marks

End of questions