GATE 2026 CY – Question 17

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. · 1 mark · Multiple choice

The species which follows 18-electron rule is (en = ethylenediamine)

$[\mathrm{Rh}(\mathrm{PPh_3})_3\mathrm{Cl}]$
$[\mathrm{Co}(\mathrm{NH_3})_6]^{2+}$
$[\mathrm V(\mathrm{CO})_6]^-$
$[\mathrm{Ni}(\mathrm{en})_3]^{2+}$

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (C) $[\mathrm V(\mathrm{CO})_6]^-$

Explanation

Count the valence electrons (the **18-electron rule**) for each species.

**A. $[\text{Rh(PPh}_3)_3\text{Cl}]$:** Rh(I) is $d^8$ (8 electrons) + 3 PPh₃ (6) + Cl⁻ (2) = 16. ✗
**B. $[\text{Co(NH}_3)_6]^{2+}$:** Co(II) $d^7$ (7) + 6 NH₃ (12) = 19. ✗
**C. $[\text{V(CO)}_6]^-$:** V(−I) is $d^6$ (6) + 6 CO (12) = 18. ✓
**D. $[\text{Ni(en)}_3]^{2+}$:** Ni(II) $d^8$ (8) + 3 en (6 donor atoms, 12) = 20. ✗

Answer **C**.