Question

In which of the following compound C-O bond length is maximum:-

• A. ${\left[Fe{\left(CO\right)}_5\right]}^{-}$
• B. $\left[Ni{\left(CO\right)}_4\right]$
• C. $\left[Cr{\left(CO\right)}_6\right]$
• D. ${\left[Mn{\left(CO\right)}_5\right]}^{+}$

Answer 1

Answer: (A)

${\left[Fe{\left(CO\right)}_5\right]}^{-}$

$CO$ are $\pi$ acceptor ligand, they accept the $e^{-}$ in ${\pi }^{\ast }$ of $CO$ and bond order decrease and so bond length increases. More the charge transfer more the $CO$ bond length
$\left[Fe\right(CO{)}_5{]}^{\ominus }$
$Fe(-I)=3d^{7}4s^2$
Maximum $e^{-}$ density so maximum back donation from $Fe$ to antibonding of $CO$. Hence maximum bond length of $CO$.