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GATE 2023 CY – Question 20

Inorganic Chemistry · Transition Metals: Coordination chemistry – structure and isomerism, theories of bonding (VBT, CFT, and MOT). Metal-metal multiple bonds. Energy level diagrams in various crystal fields, CFSE, applications of CFT, Jahn-Teller distortion. Electronic spectra of transition metal complexes: spectroscopic term symbols, selection rules, Orgel and Tanabe-Sugano diagrams, nephelauxetic effect and Racah parameter, charge-transfer spectra. Magnetic properties of transition metal complexes. Ray-Dutt and Bailar twists, Reaction mechanisms: kinetic and thermodynamic stability, associative, dissociative substitution and redox reactions. · 1 mark · Multiple choice

Formation of $[M(en)_3]^{2+}$ from $[M(H_2O)_6]^{2+}$ and three equivalents of ethylenediamine (en) is LEAST favoured when M is ______.

  1. Co
  2. Ni
  3. Cu
  4. Fe

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Correct answer: (D) Fe

Explanation

Compare divalent first-row stability using the Irving–Williams series: Fe<Co<Ni<Cu for the metals listed. Chelation benefits all four, but Fe(II) has the lowest formation stability among these alternatives.

Hence conversion to the tris-chelate is least favoured for **Fe, D**.