Question

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

Options

• Ni(CO)₄

• [Co(CO)₄]^Θ

• [Fe(CO)₄]²⁻

• [Mn(CO)₆]⁺

Answer 1

[Fe(CO)₄]²⁻

Explanation:

• [Fe(CO)₄]²⁻ is electron-rich, so it donates more electrons into the CO π* orbitals (π-backbonding).
• This weakens the C≡O bond, lowering its bond order and lengthening the C–O bond more than in the other complexes.