If in the mixed carbonyl, the other ligand is also `pi` acceptor, it would complete with the ligand CO for gaining the metal `d_pi` electron charge. The higher is the extent of back donationin CO, the lesser will be the stretching vibration frequency for C - O bond. If `PPh_3` is better `pi` - acceptor than `CO`, then answer the following.
Select correct order of stretching vibration frequency C - O bond in following molecules
I. `[Ni(CO)_4]` II. `[Ni(PF_3)(CO_3)]`

Options

A. `I gt II`
B. `I lt II`
C. `I = II`
D. cannot be predicted

Answer

B

Explanation

If `PF_3` is better `pi` aceptor ligand than CO, thenextent of back donation from `M overset (pi) to C -= O` decrease and hecne bond orderof CO will less decreses.
Stretching frequency of `C - O prop` bond order `C -= O`.
Therefore, streching frequency of `[Ni(PF_3)(CO_3)] gt [Ni(CO)_4]`