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Which of the following has longest C–O bond length? (Free C–O bond length in CO is 1.128 Å.)
A
$[Co(CO)_4]^-$
B
$[Fe(CO)_4]^{2-}$
C
$[Mn(CO)_6]^+$
D
$Ni(CO)_4$
Explanation
More negative charge on the metal → more back bonding → longer C–O.
Detailed Solution
The interaction between a filled metal orbital and a vacant orbital of CO is known as back bonding.
When the metal in a metal carbonyl complex carries a negative charge, back bonding increases; the M–C bond length decreases and the C–O bond length increases.
So $[Fe(CO)_4]^{2-}$ has the longest C–O bond: $[Mn(CO)_6]^+ < Ni(CO)_4 < [Co(CO)_4]^- < [Fe(CO)_4]^{2-}$
When the metal in a metal carbonyl complex carries a negative charge, back bonding increases; the M–C bond length decreases and the C–O bond length increases.
So $[Fe(CO)_4]^{2-}$ has the longest C–O bond: $[Mn(CO)_6]^+ < Ni(CO)_4 < [Co(CO)_4]^- < [Fe(CO)_4]^{2-}$