1. Ni(CO)₄ 2. [Co(CO)₄]^(ominus) 3. [Fe(CO)₄]²⁻ 4. [Mn(CO)₆]⁺

Topic: Valence bond approach· Coordination Compounds

Which of the following has longest C-O bond length ? (Free C-O bond length in CO is 1.128\ .)

1. \(Ni(CO)_4\)

2. \([Co(CO)_4]^{\ominus}\)

3. \([Fe(CO)_4]^{2-}\)

4. \([Mn(CO)_6]^{+}\)

Doubtly Asked 30 Oct 2026 NEET UG 2016 Phase 1 Practice test neet Save WhatsApp Report

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Answer: 3. \([Fe(CO)_4]^{2-}\)

Key idea: in metal carbonyls, back bonding from metal d orbitals into the CO \(\pi^*\) orbital weakens the C-O bond and lengthens it. More electron density on the metal means more back bonding and a longer C-O bond.

The species are isoelectronic (metal with 18 electrons), but the negative charge is highest on \([Fe(CO)_4]^{2-}\), so its metal is the most electron-rich. A positive charge (as in \([Mn(CO)_6]^+\)) reduces back bonding. So \([Fe(CO)_4]^{2-}\) has the longest C-O bond.

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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Which of the following has longest C-O bond length ? (Free C-O bond length in CO is 1.128\ .)

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