Consider following 4 complexes :

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

(R) $\left[\mathrm{Fe}(\mathrm{CO}{)}_5\right]$ (S) $\left[\mathrm{Fe}(\mathrm{CO}{)}_4\left({\mathrm{PEt}}_3\right)\right]$

Choose the correct statement.

Order of 1t accepting tendency :

$\mathrm{CO}>{\mathrm{PPh}}_3>{\mathrm{PEt}}_3$

${\mathrm{PEt}}_3$ is a better a-donor than ${\mathrm{PPh}}_3$. Thus, ${\mathrm{PEt}}_3\uparrow$

to CO thus, decreasing CO bond strength or stretching frequency v. Also, M-C bond becomes stronger in (P).

Similarly, $\sigma -$donor, ${\mathrm{PEt}}_3\uparrow$ electron density on Fe which back donates it to CO, decreasing C-0 bond strength or v. 

$\Rightarrow R$ has longer stretching frequency than 'S' and longer M-C bond than 'S'.