If in the mixed carbonyl the other ligand is also `pi` acceptor it would compete with the ligand `CO` for gaining the metal `d_(pi)` electron charge The higher is the extent of back donation in `CO` , the lesser will be the stretching vibration frequency for `C-O` bond If `PP_(3)` is better pi-acceptor than `CO` then answer the following
Select the correct order of stretaching vibration frequency `C-O` bond in following molecules
(I) `[Ni(CO))_(4)]`
(II) `[Ni(PF_(3))(CO)_(3)]`
(a) `I=II`
`I ltII`
(c ) `I =II`
(d) cannot be predicated .

Answer

B

Explanation

If `PF_(3)` is a better pi-acepptor ligand than `CO` then extent of back donation from `Moverset(pi)rarrC-=O` decreases and hence bond order of `CO` will less decrease Stretching frquency of `C-O` and prop bond order of `C-=O`
Therefore stretching frequency of
`[Ni(PF_(3))(CO_(3))]gt[Ni(CO)_(4)]` .