Consider the following complexes 

$\text{ (P) }\left[\mathrm{Cr}(\mathrm{CO}{)}_5\left({\mathrm{PF}}_3\right)\right]$

$\left(\mathrm{Q}\right)\left[\mathrm{Cr}(\mathrm{CO}{)}_5\left({\mathrm{PCl}}_3\right)\right]$

Select the correct options.

Due to back donation C-O bond length in given compounds is longer than free CO molecule. ${\mathrm{PF}}_3$ is better n-acceptor than CO, hence back donation is more favoured towards ${\mathrm{PF}}_3$ rather than CO and ${\mathrm{PF}}_3$ is better n-acceptor than ${\mathrm{PCl}}_3$ hence $\mathrm{\pi }$-donation is more favorable in (P) than (Q). Hence, CO length in (P) is shorter than (Q). Due to less n-donations towards CO or C bond is longer in (P) than (Q).

Hence correct option is (a, b, c, d).