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Which of the following carbonyls will have the strongest C–O bond?
A
$[V(CO)_6]^-$
B
$Fe(CO)_5$
C
$[Mn(CO)_6]^+$
D
$Cr(CO)_6$
Detailed Solution
In metal carbonyls the metal donates electron density from its filled d orbitals into the empty $\pi^*$ orbital of CO (back bonding).
Electrons entering the antibonding $\pi^*$ orbital of CO weaken the C–O bond; the greater the back donation, the lower the C–O bond order.
Back donation increases with the electron density on the metal: a negative charge on the complex increases it and a positive charge decreases it.
$[V(CO)_6]^-$ has the most electron-rich metal (maximum back bonding, weakest C–O bond); the neutral carbonyls $Cr(CO)_6$ and $Fe(CO)_5$ are intermediate.
$[Mn(CO)_6]^+$ has a positively charged metal centre, so back donation is the least and the C–O bond retains the most triple bond character.
Hence $[Mn(CO)_6]^+$ has the strongest C–O bond.
Electrons entering the antibonding $\pi^*$ orbital of CO weaken the C–O bond; the greater the back donation, the lower the C–O bond order.
Back donation increases with the electron density on the metal: a negative charge on the complex increases it and a positive charge decreases it.
$[V(CO)_6]^-$ has the most electron-rich metal (maximum back bonding, weakest C–O bond); the neutral carbonyls $Cr(CO)_6$ and $Fe(CO)_5$ are intermediate.
$[Mn(CO)_6]^+$ has a positively charged metal centre, so back donation is the least and the C–O bond retains the most triple bond character.
Hence $[Mn(CO)_6]^+$ has the strongest C–O bond.