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A2 · Practice questions · Complex ions and ligands

Count the dative bonds.

Six original Cambridge-style questions on complex ions and ligands.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Ligand, coordination number, shape, charge.

01
Recall
[3 marks]

Define the terms (a) ligand, (b) complex, (c) coordination number.

  • (a) A molecule or ion with a lone pair that forms a dative covalent bond to a central metal atom or ion. ✓
  • (b) A central metal atom or ion surrounded by ligands bonded to it by dative bonds. ✓
  • (c) The number of dative bonds formed between the ligands and the central metal atom or ion. ✓
02
Analysis
[3 marks]

For [Co(en)2Cl2]+, give (a) the oxidation state of cobalt, (b) the coordination number, (c) the shape.

  • (a) en is neutral and each Cl− is −1, so x − 2 = +1, x = +3. ✓
  • (b) 2 × 2 (en) + 2 × 1 (Cl) = 6. ✓
  • (c) octahedral. ✓
03
Explain
[2 marks]

Explain why 1,2-diaminoethane is described as a bidentate ligand.

  • Each of its two nitrogen atoms has a lone pair. ✓
  • Both lone pairs form dative bonds to the same metal ion, so one ligand forms two bonds. ✓
04
Recall
[3 marks]

Draw and name the shape of (a) [Cu(H2O)6]2+, (b) [CuCl4]2−, (c) [Ag(NH3)2]+, stating bond angles.

  • (a) octahedral, 90°, six O donor atoms around Cu. ✓
  • (b) tetrahedral, 109.5°. ✓
  • (c) linear, 180°. ✓
05
Analysis
[2 marks]

EDTA4− forms a 1 : 1 complex with Fe3+. Give the formula and charge of the complex and its coordination number.

  • [Fe(EDTA)]−: +3 + (−4) = −1. ✓
  • EDTA4− is hexadentate, so the coordination number is 6 (octahedral). ✓
06
Explain
[2 marks]

Suggest why Cu2+ forms a six-coordinate complex with water but a four-coordinate complex with chloride ions.

  • Chloride ions are much larger than water molecules. ✓
  • Only four Cl− ions fit around the Cu2+ ion (and they repel each other), giving a tetrahedral complex. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.