UPSC Civil Services (Mains), Chemistry optional · 2022 · Paper I

Q6(e). Origin of the yellow colour of aqueous FeCl₃

10 marks
Topic: Coordination chemistry
Answered

Question

Aqueous solution of FeCl₃ is bright yellow and not pale-violet like other metal ions having high-spin d⁵ configuration. Discuss the origin of the colour.

Answer in brief

Not a d–d colour: it is ligand-to-metal charge transfer (OH⁻, Cl⁻ → Fe³⁺) in the hydroxido and chlorido complexes [Fe(H₂O)₅(OH)]²⁺ and [FeCl(H₂O)₅]²⁺ of the solution.

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Topic notes: electronic spectra of complexes: term symbols, selection rules, Orgel diagrams and charge transfer, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coverscharge transferLMCTferric chloride colourhigh-spin d5hydrolysis of Fe3+spin-forbidden transition

All parts of Question 6

6(a) Metal–metal bonds in Cp₂Fe₂(CO)₄; structure of Bi₅³⁺10 marks
6(b) Pale colours and line-like spectra of lanthanide ions10 marks
6(c) Neutral mixed sandwich compound of a d⁶ centre10 marks
6(d) Separating XeF₆; diborane with NH₃ and with Me₃N10 marks
6(e) Origin of the yellow colour of aqueous FeCl₃10 marks