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GATE 2025 CY – Question 37

Inorganic Chemistry · Organometallics: 18-Electron rule; metal-alkyl, metal-carbonyl, metal-olefin and metal-carbene complexes and metallocenes. Fluxionality in organometallic complexes. Types of organometallic reactions. Homogeneous catalysis – Hydrogenation, hydroformylation, methanol to acetic acid process, olefin metathesis and Wacker oxidation. Heterogeneous catalysis – Fischer-Tropsch reaction, Ziegler-Natta polymerization. · 2 marks · Multiple choice

The homogeneous catalyst whose metal ion does NOT undergo either oxidation or reduction in any of the steps during the hydrogenation of terminal olefins is

  1. $\mathrm{RhCl(PPh_3)_3}$
  2. $\mathrm{HRuCl(PPh_3)_3}$
  3. $[\mathrm{Ir(COD)(PCy_3)(Py)}]^+\mathrm{PF_6^-}$ (COD = cyclooctadiene)
  4. $[\mathrm{Rh(COD)(PPh_3)_2}]^+\mathrm{PF_6^-}$ (COD = cyclooctadiene)

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Correct answer: (B) $\mathrm{HRuCl(PPh_3)_3}$

Explanation

In hydrogenation, a catalyst can work in two ways:
- **Oxidative addition of $H_2$** to the metal (the metal's oxidation state goes up by 2), followed by olefin insertion and reductive elimination (oxidation state goes back down).
- **Hydride route (no change of oxidation state):** a metal hydride adds to the olefin by insertion, and the alkyl is cleaved by $H_2$ in a sigma-bond metathesis ("hydrogenolysis") step that regenerates the M–H.

Check the catalysts:
- **$\text{RhCl(PPh}_3)_3$ (Wilkinson's):** Rh(I) is oxidised to Rh(III) by $H_2$. ✗
- **$\text{HRuCl(PPh}_3)_3$:** Ru(II) hydride, which hydrogenates through olefin insertion into Ru–H and hydrogenolysis of the Ru–alkyl by $H_2$. The Ru stays Ru(II) throughout. ✓
- **Crabtree's catalyst $[\text{Ir(COD)(PCy}_3)(\text{Py})]^+$ and $[\text{Rh(COD)(PPh}_3)_2]^+$:** both use oxidative addition of $H_2$ and reductive elimination. ✗

Answer **$\text{HRuCl(PPh}_3)_3$** (B).