1.

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.

Answer»

P will havel LOWER C-O stretching frequency and a longer metal carbon bond than Q.
P will have lower C-O stretching frequency and a SHORTER metal carbon bond than Q.
R has higher C-O stretching frequency and a longer metal carbon bond than S.
S has higher C-O stretching frequency and a longer metal carbon bond than R.

Solution :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'.


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