Question

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

Answer 1

your question is incomplete. but if following are the options then;

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

from these options, the correct answer is option no 2 that is [Fe(CO)4] is correct, because it has the longest carbon-oxygen bond length.

hope so this will help you in your question...

Answer 2

[Fe(CO)4]2- has the most extended CO bond length.

It is explained as whenever the negative charge of the metal carbonyl complex increases back PI bonding also increases with that while the bond length of the metal-carbon bond decreases.

It is because the more significant the negative charge on the carbonyl complex it will be more accessible for the metal to permit its electrons to participate in the back bonding.