Arrange the following complex in increasing order of stretching vibrational frequency of $\mathrm{C}-\mathrm{O}$ bond.

(P) $\left[{\left({\mathrm{PPh}}_3\right)}_3\mathrm{Mo}(\mathrm{CO}{)}_3\right]$  (Q)  $\left[{\left({\mathrm{Ph}}_2\mathrm{PCl}\right)}_3\mathrm{Mo}(\mathrm{CO}{)}_3\right]$

(R) $\left[{\left({\mathrm{PhPCl}}_2\right)}_3\mathrm{Mo}(\mathrm{CO}{)}_3\right]$ (S) $\left[{\left({\mathrm{PCl}}_3\right)}_3\mathrm{Mo}(\mathrm{CO}{)}_3\right]$

The $\mathrm{\sigma }$-donating ability and $\mathrm{\pi }$-acceptor ability of phosphines are inversely correlated in the sense that electron-rich phosphines, such as ${\mathrm{PMe}}_3$, are good $\mathrm{\sigma }$-donors and poor $\mathrm{\pi }$-acceptors, whereas electron-poor phosphines, such as ${\mathrm{PF}}_3$, are poor $\mathrm{\sigma }$-donors and good $\mathrm{\pi }$-acceptors. Thus, $\mathrm{\pi }$-accepting tendency order:

${\mathrm{PCl}}_3>{\mathrm{PhPCl}}_2>{\mathrm{Ph}}_2\mathrm{PCl}>{\mathrm{Ph}}_3\mathrm{P}$

$\left.\left.\begin{array}{c}\mathrm{C}-\mathrm{O}\\ \text{ B.S.more }\end{array}\right\}\Rightarrow \begin{array}{cc}{v}_{ca}& \mathrm{C}-\mathrm{O}\\ \text{ higher }& \text{ B.S.less }\end{array}\right\}{\Rightarrow }^v\text{ lower }$

Hence, option D is correct.