In which of the following compound C--O bond length is maximum
(1) [Fe(CO)]-5
(2) [Ni(CO)4]

Dear Student,

In Metal Carbonyls the M-C bond possess both σ and π character. The donation of electrons on the carbonyl carbon into the vacant orbital of the metal forms the M-C 'σ' bond and the M-C 'π' bond is formed by the donation of the pair of electrons from the filled d-orbitals of metal into the vacant antibonding π* orbital of the carbonyls.

This affects the bond length in the complex thus formed. The M-C bond shortens and the C-O bond lengthens.

Since Nickel has more number of d-electrons in its orbit than Iron, Ni-C π bond in Ni(CO)4 will takes place more strongly and hence the C-O bond lengthens to a greater extent than in the case of Fe(CO)5. So, Option (2) is correct.

Best Wishes !

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