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GATE 2024 CY – Question 38

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 heptacity of allyl and Cp and the ligation mode of NO in the thermodynamically stable complexes $[(\eta^x\text{-allyl})\mathrm{Ru(CO)_2(NO)}]$ and $[(\eta^y\text{-Cp})\mathrm{Ru(CO)_2(NO)}]$, respectively, are (The heptacity of allyl and Cp are denoted by $\eta^x$ and $\eta^y$, respectively.)

  1. $(\eta^3,\text{ NO-bent})$ and $(\eta^5,\text{ NO-linear})$
  2. $(\eta^3,\text{ NO-linear})$ and $(\eta^5,\text{ NO-bent})$
  3. $(\eta^1,\text{ NO-bent})$ and $(\eta^3,\text{ NO-bent})$
  4. $(\eta^1,\text{ NO-bent})$ and $(\eta^5,\text{ NO-linear})$

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Correct answer: (B) $(\eta^3,\text{ NO-linear})$ and $(\eta^5,\text{ NO-bent})$

Explanation

Use the **18-electron rule** for each complex, with the neutral-ligand counting method.

**Allyl complex** $[(\eta^x\text{-allyl})\text{Ru(CO)}_2(\text{NO})]$:
- An $\eta^3$-allyl donates 3 electrons.
- A **linear** NO (NO⁺-like) donates 3 electrons.
- Total: $8+3+4+3=18$ ✓. So allyl is $\eta^3$ and NO is **linear**.

**Cp complex** $[(\eta^y\text{-Cp})\text{Ru(CO)}_2(\text{NO})]$:
- An $\eta^5$-Cp donates 5 electrons.
- A **bent** NO (NO⁻-like) donates 1 electron.
- Total: $8+5+4+1=18$ ✓. So Cp is $\eta^5$ and NO is **bent**.

Answer: **($\eta^3$, NO-linear) and ($\eta^5$, NO-bent)** (option B).