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In the complex [Fe(CO)5], the Fe–C bond is best described as having which type of bonding?

1. predominantly ionic nature
2. only sigma bonding
3. only pi bonding
4. both sigma and pi bonding

Correct answer: both sigma and pi bonding

Solution

In Fe(CO)5 the M-C bond is synergic: CO donates its lone pair into a metal orbital (sigma bond) while filled metal d-orbitals back-donate into the CO pi* orbital (pi bond). Hence the Fe-C bond has both sigma and pi character.

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