Complex ions and ligandsEdexcel A-Level Chemistry: Subtopic test

1. 1
   A student dissolves iron(II) sulfate in water. The iron(II) ions are present as the pale green complex ion [Fe(H₂O)₆]²⁺.

   (a)

   What is the coordination number of iron in [Fe(H₂O)₆]²⁺?

   [1 mark]

   • A2
   • B4
   • C6
   • D12

   (b)

   Which statement describes the bond between a water molecule and the Fe²⁺ ion?

   [1 mark]

   • AA dative covalent bond in which oxygen donates a lone pair to the Fe²⁺ ion
   • BAn ionic bond between Fe²⁺ and the whole water molecule
   • CA covalent bond with one electron donated by each atom
   • DA hydrogen bond between a hydrogen atom of water and the Fe²⁺ ion

   (c)

   Explain why water acts as a monodentate ligand.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Cis-platin, Pt(NH₃)₂Cl₂, is a square planar complex used in chemotherapy. Its isomer, trans-platin, has the same formula but is not used as a drug.

   (a)

   What is the oxidation state of platinum in cis-platin?

   [1 mark]

   • A+4
   • B0
   • C−2
   • D+2

   (b)

   Which row gives the shape of cis-platin and the angle between adjacent ligands?

   [1 mark]

   • ATetrahedral, 109.5°
   • BSquare planar, 90°
   • COctahedral, 90°
   • DLinear, 180°

   (c)

   Explain why cis-platin is supplied for cancer treatment as a single isomer rather than as a mixture with trans-platin.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Nickel(II) ions form the green complex ion [Ni(H₂O)₆]²⁺ in water. Chemists can replace the water ligands with ethane-1,2-diamine, NH₂CH₂CH₂NH₂ (en), or with the ethylenediaminetetraacetate ion, EDTA⁴⁻.

   (a)

   Ethane-1,2-diamine is a bidentate ligand and forms the complex ion [Ni(NH₂CH₂CH₂NH₂)₃]²⁺. Explain what is meant by bidentate and deduce the coordination number of nickel in this complex.

   [3 marks]

   (b)

   EDTA⁴⁻ reacts with [Ni(H₂O)₆]²⁺ in a 1 : 1 ratio to give a complex ion in which the nickel(II) ion is surrounded by one EDTA⁴⁻ ion only. (i) Write the ionic equation for the reaction. (ii) Deduce the overall charge on the product and (iii) the shape of the complex ion, explaining your answer.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Iron(II) forms complexes in living organisms and in the laboratory. In the blood, iron(II) at the centre of haemoglobin binds oxygen reversibly. In the laboratory, iron(II) ions in water are present as [Fe(H₂O)₆]²⁺, and iron(II) can also form a complex with EDTA⁴⁻.

   (a)

   Carbon monoxide is poisonous because it binds to haemoglobin. Explain, in terms of ligands and bonding, how carbon monoxide prevents haemoglobin from carrying oxygen.

   [6 marks]

   (b)

   Compare [Fe(H₂O)₆]²⁺ with the EDTA complex of iron(II), [Fe(EDTA)]²⁻. Refer to the type of ligand, coordination number, shape and overall charge, and show how the charge on [Fe(EDTA)]²⁻ is deduced.

   [6 marks]

   Total for question 4: 12 marks

End of questions