OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2020: Question 15

1 mark · Medium difficulty · Multiple Choice

Identify the correct statements regarding the isomerism and coordination number of the complex ion [Co(NH2CH2CH2NH2)3]2+.

Practise this question

Question

Multiple-choice question 15 asking to evaluate statements about the complex ion [Co(NH2CH2CH2NH2)3]2+: statement 1 says it has cis and trans isomers, statement 2 says it has optical isomers, and statement 3 says it has six-fold coordination. Four options are given: A (1, 2 and 3), B (Only 1 and 2), C (Only 2 and 3), and D (Only 1). A box for the answer is provided at the bottom with a mark allocation of [1].
Question text

15 Which statement(s) for the complex ion [Co(NH CH CH NH ) ]2+ is/are correct?

22 2 2 3

1 It has cis and trans isomers.

2 It has optical isomers.

3 It is six-fold coordination.

A 1, 2 and 3

B Only 1 and 2

C Only 2 and 3

D Only 1

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme table showing that the correct answer for question 15 is C.

15 C 1 1.1

Total 15

SECTION B

How to answer it

Complex Ion Isomerism and Coordination Number

What this question tests

This question assesses your understanding of transition metal complex chemistry, specifically determining coordination numbers with multidentate ligands (such as en or NH₂CH₂CH₂NH₂ ), recognizing stereoisomerism (cis/trans geometric isomerism versus optical isomerism), and spatial visualization of octahedral complexes.

Question 15 Multiple Choice Analysis

Correct Answer: C (Only 2 and 3)

✅ Correct Answer Breakdown

The correct option is C, meaning statements 2 and 3 are correct, while statement 1 is incorrect.

  • Statement 1: FALSE. A tris-chelate octahedral complex like [Co(NH₂CH₂CH₂NH₂)]²⁺ does not form cis and trans geometric isomers.
  • Statement 2: TRUE. The complex is chiral and exhibits optical isomerism (non-superimposable mirror images).
  • Statement 3: TRUE. There are three bidentate ligands, each forming 2 coordinate bonds, giving a coordination number of 6 (six-fold coordination).

💡 Key Knowledge

  • Ligand classification: NH₂CH₂CH₂NH₂ is 1,2-diaminoethane (often abbreviated as en ), which is a bidentate ligand containing two nitrogen atoms with lone pairs.
  • Coordination Number Formula: (Number of bidentate ligands × 2) = 3 × 2 = 6.
  • Octahedral Stereoisomerism: Complexes of the type [M(bidentate)₃] do not have cis/trans isomers because all three bidentate chelating rings are equivalent, but they do exist as a pair of optical enantiomers (delta Δ and lambda Λ forms).

🧠 Exam Technique

When evaluating multi-statement multiple-choice questions:

  • Evaluate the easiest statement first to eliminate options quickly. Knowing that en is bidentate immediately confirms statement 3 (six-fold coordination), eliminating options B and D.
  • Be careful with canned definitions: do not automatically associate multidentate ligands with cis/trans isomerism unless they are monodentate mixes (like [M(bidentate)₂(monodentate)₂] ).

❌ Common Errors

  • Confusing Isomer Types: Many students see a complex with multiple ligands and automatically assume it has cis and trans isomers without drawing or visualizing the 3D octahedral arrangement.
  • Miscounting Coordination Sites: Treating 1,2-diaminoethane as a monodentate ligand because it looks similar to ammonia ( NH₃ ). Remember to check the carbon backbone length!
Mark Scheme Note: 1 mark awarded for selecting C. No other combination is credited.

Topics

Module 5: Physical chemistry and transition elements · 5.3 Transition elements

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.