Consider the following complexes

$\text{ (P) }\left[\mathrm{Cr}(\mathrm{CO}{)}_x\right]$      $\left(Q\right)\left[\mathrm{Cr}(\mathrm{CO}{)}_{(x-1)}{\mathrm{PF}}_3\right]$

If ${\mathrm{PF}}_3$ is better $\mathrm{\pi }$ accepter than $\mathrm{CO}$, what will be the order of bond length of $\mathrm{CO}$in complexes (P) and (Q).

Due to the fact that ${\mathrm{PF}}_3$ is a greater "-acceptor" than $\mathrm{CO}$, back bonding occurs in place of $\mathrm{CO}$.So, $\mathrm{CO}$B.L. gets shorter.

If ${\mathrm{PF}}_3$ is better $\mathrm{\pi }$ accepter than $\mathrm{CO}$, what will be the order of bond length of $\mathrm{CO}$in complexes (P) and (Q). then $\mathrm{P} > \mathrm{Q}$.