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

Options

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

Answer

B

Explanation

As negative charge on metal carbonyl complex increases, back `pi`-bonding increases and hence bond length of C- O bond increases while bond length of metal-carbon bond decreases.
Hence, `[Fe(CO)_(4)]^(2-)` has longest C-O bond length among the given complexes.
The correct order of bond length of the given complexes is
`[Mn(CO)_(6)]^(+) lt [Ni(CO)_(4)] lt [Co(CO)_(4)]^(-) lt [Fe(CO)_(4)]^(2-)`