Question

Among the following metal carbonyls, the C-O bond order is lowest in:

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

Answer 1

The correct option is B $\left[Fe\right(CO{)}_5]$

The complex in which back-bonding is maximum will have the lowest bond order.
Clearly, maximum back-bonding happens in option (c) i.e. in $\left[Fe\right(CO{)}_5],$ therefore CO bond order is lowest here.