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Which of the following has longest C–O bond length? (Free C–O bond length in CO is 1.128Å)

1. [Ni(CO)4]
2. [Co(CO)4]+
3. [Fe(CO)5]2−
4. [Mn(CO)6]+

Correct answer: [Fe(CO)5]2−

Solution

The bond length of C–O increases as the back-donation of electron density from the metal to the antibonding orbitals of CO increases. In [Fe(CO)5]2−, the negative charge on the complex enhances back-donation, leading to the longest C–O bond length.

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