Question 25 of 34intermediate🔍 AnalyzeShort Answer2 marks

[Fe(CN)6]4- and [Fe(H2O)6]2+ are of different colours in dilute solutions. Why?

Correct Answer

The colour of coordination compounds depends on the crystal field splitting energy (Δo), which is influenced by the nature of the ligand attached to the central metal ion.

In [Fe(CN)₆]⁴⁻, CN⁻ is a strong field ligand that causes large crystal field splitting, whereas in [Fe(H₂O)₆]²⁺, H₂O is a weak field ligand that causes smaller splitting. The different magnitudes of Δo result in absorption of different wavelengths of light from the visible spectrum.

Since different wavelengths are absorbed by the two complexes, different complementary colours are observed in their dilute solutions.

Exercise: EXERCISES | Q: 5.21 | (Page Page 22)
For More Understanding

Explanation

The textbook context discusses how the influence of the ligand on the colour of a complex is significant. It mentions that in the absence of ligand, crystal field splitting does not occur and the substance is colourless. The context also references inner orbital complexes (like [Fe(CN)₆]³⁻ with d²sp³ hybridisation) which form with strong field ligands, and outer orbital complexes (like [FeF₆]³⁻ with sp³d² hybridisation) which form with weak field ligands. This distinction between strong and weak field ligands directly relates to the magnitude of crystal field splitting energy (Δo), which determines the wavelength of light absorbed and hence the colour observed.

Solution Steps

  1. Step 1: Identify that colour depends on crystal field splitting energy (Δo)

  2. Step 2: Recognize that CN⁻ is a strong field ligand causing large Δo, while H₂O is a weak field ligand causing smaller Δo

  3. Step 3: Conclude that different Δo values lead to absorption of different wavelengths, resulting in different colours

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