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

Options

A. `[Fe(CO)_(4)]^(2-)`
B. `[Mn(CO)_(6)]^(+)`
C. `[Ni(CO)_(4)]`
D. `[Co(CO)_(4)]^(-)`

Answer

A

Explanation

The greater the negative charge on the carbonyl complex, the easier it would be for the metal to allow its electrons to participate in the back bonding. As a result, higher would be the `M-C` bond order and simultaneously, there would be greater reduction in the `C-O` bond order and hence larger bond length. Thus, `[Fe(CO)_(4)]^(2-)` has the lowest `C-O` bond order and, therefore, the longest bond length.