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Chemistry · JEE Advanced only

Metal Carbonyls (JEE Advanced)

Not in the JEE Main syllabus, but it is in JEE Advanced. Study time: about 2 to 3 hours. Spend about 1 hour understanding the bonding and 18-electron rule with examples, then 1 hour practicing formula predictions for different metals, and 30 minutes on polynuclear carbonyls and exceptions..

Last updated: 26 October 2026

Direct answer

Metal carbonyls are coordination compounds where carbon monoxide (CO) acts as a ligand bonded to transition metals in low oxidation states, typically zero.

Metal carbonyls are coordination compounds where carbon monoxide (CO) acts as a ligand bonded to transition metals in low oxidation states, typically zero. These compounds are important in industrial processes like hydroformylation and serve as precursors for synthesizing other organometallic compounds. The bonding involves both sigma donation from CO to metal and pi back-bonding from metal to CO.

Official syllabus

What the JEE Advanced syllabus says

“Metal carbonyls”

Source: JEE Advanced 2026 Information Bulletin, Annexure-I. Closest JEE Main topic: importance of coordination compounds (in qualitative analysis, extraction of metals and in biological systems).

JEE Main covers coordination compounds broadly but skips metal carbonyls entirely. For Advanced, you need to understand their structures, bonding, the 18-electron rule, and how to predict formulas and shapes.

Key ideas

What you must understand

  • CO is a strong field ligand that bonds via sigma donation (C donates lone pair to metal) and pi back-bonding (metal d-electrons go into antibonding orbitals of CO)
  • Most metal carbonyls follow the 18-electron rule: the metal achieves 18 electrons in its valence shell for stability, similar to noble gas configuration
  • Metal carbonyls can be mononuclear like Ni(CO)₄ and Fe(CO)₅, or polynuclear with metal-metal bonds like Mn₂(CO)₁₀ and Co₂(CO)₈
  • Synergic bonding: sigma donation strengthens the M-C bond while pi back-bonding weakens the C-O bond, which can be detected by decrease in CO stretching frequency in IR spectroscopy
  • Common geometries: Ni(CO)₄ is tetrahedral, Fe(CO)₅ is trigonal bipyramidal, Cr(CO)₆ is octahedral
Formulas

Formulas and key facts

  • 18-electron rule: Metal valence electrons + (2 × number of CO ligands) = 18
  • Ni(CO)₄: Ni has 10 e⁻, 4 CO give 8 e⁻, total = 18 e⁻
  • Fe(CO)₅: Fe has 8 e⁻, 5 CO give 10 e⁻, total = 18 e⁻
  • Cr(CO)₆: Cr has 6 e⁻, 6 CO give 12 e⁻, total = 18 e⁻
  • V(CO)₆: V has 5 e⁻, 6 CO give 12 e⁻, total = 17 e⁻ (paramagnetic, exception)
  • For dinuclear carbonyls: each M-M bond contributes 1 e⁻ to each metal
  • Mn₂(CO)₁₀: Each Mn has 7 e⁻ + 10 e⁻ from 5 CO + 1 e⁻ from Mn-Mn bond = 18 e⁻
Avoid

Common mistakes

  • Forgetting that CO donates 2 electrons per ligand, not 1. Each CO is a two-electron donor through its carbon lone pair.
  • Ignoring metal-metal bonds in polynuclear carbonyls. In Mn₂(CO)₁₀, the Mn-Mn bond is essential for each Mn to reach 18 electrons.
  • Assuming all metal carbonyls are diamagnetic. V(CO)₆ has 17 electrons and is paramagnetic.
  • Confusing oxidation state with electron count. In most carbonyls, the metal is in zero oxidation state, but you count its group number electrons for the 18-electron rule.
Worked example

A JEE Advanced-level example

Question. Predict the formula of the simplest mononuclear carbonyl of cobalt (Co, atomic number 27) that obeys the 18-electron rule. If such a compound is unstable, suggest the formula of a stable dinuclear carbonyl.

Show solution
  1. Step 1: Find valence electrons of Co. Co is in group 9, so it has 9 valence electrons.
  2. Step 2: Apply 18-electron rule for mononuclear carbonyl. Need 18 - 9 = 9 electrons from CO ligands. Since each CO gives 2 electrons, we need 9/2 = 4.5 CO ligands.
  3. Step 3: Since 4.5 is not a whole number, a simple mononuclear Co(CO)ₓ cannot satisfy the 18-electron rule. Co(CO)₄ gives only 17 electrons (unstable, paramagnetic).
  4. Step 4: For a dinuclear carbonyl Co₂(CO)ₓ, each Co can form a Co-Co bond, contributing 1 electron to each Co.
  5. Step 5: Each Co now needs: 18 - 9 - 1 = 8 electrons from CO, meaning 4 CO per Co atom. Total CO ligands = 8.
  6. Step 6: The stable dinuclear carbonyl is Co₂(CO)₈, where each Co has 9 + 1 + 8 = 18 electrons.

Answer: Mononuclear Co(CO)₄ is unstable (17 e⁻). The stable dinuclear carbonyl is Co₂(CO)₈.

Solved twice independently; both solutions agree.

FAQs

Frequently asked questions

Is Metal Carbonyls in the JEE Main syllabus?

No. It is in the JEE Advanced 2026 syllabus only. The closest JEE Main topic is importance of coordination compounds (in qualitative analysis, extraction of metals and in biological systems).

How long does Metal Carbonyls take to prepare for JEE Advanced?

About 2 to 3 hours. Spend about 1 hour understanding the bonding and 18-electron rule with examples, then 1 hour practicing formula predictions for different metals, and 30 minutes on polynuclear carbonyls and exceptions., if you already know the related JEE Main topic.

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