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Question
[Fe(CN)6]4− and [Fe(H2O)6]2+ are of different colours in dilute solutions. Why?
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Solution 1
The colour of a particular coordination compound depends on the magnitude of the crystal-field splitting energy, Δ. This CFSE in turn depends on the nature of the ligand. In case of [Fe(CN)6]4− and [Fe(H2O)6]2+, the colour differs because there is a difference in the CFSE. Now, CN− is a strong field ligand having a higher CFSE value as compared to the CFSE value of water. This means that the absorption of energy for the intra d-d transition also differs. Hence, the transmitted colour also differs.
Solution 2
In [Fe(CN)6]4−, CN− is a strong field ligand; hence, pairing of electrons takes place.
In [Fe(H2O)6]4−, H2O is a weak ligand; hence, pairing does not take place.
Because of their differing natures, these ligands exhibit varying magnitudes of crystal field splitting, resulting in different wavelengths and colours.
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