Question

Correct sequence of $CO$ bond order in given compounds is:
(P) $Fe(CO{)}_5$ (Q) $CO$ (R) $H_{3}B\leftarrow CO$ (S) $\left[Mn\right(CO){+}_5{]}^{-}$.

• A. $P>R>S>Q$
• B. $S>P>R>Q$
• C. $Q>S>P>R$
• D. $R>Q>P>S$

Answer 1

The correct option is D $R>Q>P>S$

$CO$ bond order $\propto \frac{1}{\text{Extent of back bonding}(M\to C)}$

Correct sequence of $CO$ bond order:

$\underset{(CObondorder>3.0)}{H_{3}B\leftarrow CO}>\underset{B.O.=3.0}{CO}>\underset{Cobondorder<3.0}{Fe(CO{)}_5}>\underset{(CObondorder<3.0)}{\left[Mn\right(CO{)}_5{]}^{-}}$.

thus, D is the correct answer.