Consider following 4 complexes :
`(P) [Cr(CO)_(5)(Pet_(3))]" "(Q) [Cr(CO)_(5)(PPh_(3)]`
`(R ) [Fe(CO)_(5)]" "(S) [Fe(CO)_(4)(Pet_(3))]`
Choose the correct statement.

Options

A. P will havel lower C-O stretching frequency and a longer metal carbon bond than Q.
B. P will have lower C-O stretching frequency and a shorter metal carbon bond than Q.
C. R has higher C-O stretching frequency and a longer metal carbon bond than S.
D. S has higher C-O stretching frequency and a longer metal carbon bond than R.

Answer

B, C

Explanation

Order of `pi` accepting tendency :
`Co gt PPh_(3) gt Pet_(3)`
`Pet_(3)` is a better `sigma`-donor than `PPh_(3)`. Thus, `Pet_(3)uarr` the electron density on C.A. which back donates it to CO thus, decreasing CO bond strength or stretching frequency u. Also, M-C bond becomes stronger in (P).
. Similarly, `sigma`-donor, `PEt_(3)uarr` electron density on Fe which back donates it to CO, decreasing C-O bond strength or v.
`implies R` has longer stretching frequency than 'S' and longer M-C bond than 'S'.