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

Options

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

Answer

D

Explanation

`F to C .........O`
Due to back bonding between metal -carbon bond, the length of C-O increases `("B.O of M-C "uparrow" B.O of C-C "downarrow" B.L of (C-O" uparrow))`. Higher is the negative charge on metal, higher is the back bonding (synergic effect). So bond length is higher. Hence `[Fe(CO)_(4)]^(2-)` is correct. More is positive charge on metal more is strength of C-O bond and less is bond length.