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 .

Explanation

To solve the problem, we need to analyze the effect of the ligands on the stretching vibration frequency of the C-O bond in the given coordination complexes.

### Step-by-Step Solution:

1. **Understanding Ligands and Back Donation**:
- Carbon monoxide (CO) is a strong π-acceptor ligand. This means it can accept electron density from the metal's d-orbitals through back donation.
- Phosphorus trifluoride (PF₃) is also a π-acceptor ligand, but it is stated that PF₃ is a better π-acceptor than CO.

2. **Effect of Back Donation on C-O Bond**:
- The extent of back donation from the metal to the CO ligand affects the bond order of the C-O bond.
- If back donation is more significant, the C-O bond order decreases, leading to a lower stretching frequency of the C-O bond.

3. **Comparing the Two Complexes**:
- For the complex [Ni(CO)₄], CO is the only ligand, and back donation occurs fully, resulting in a certain stretching frequency for the C-O bond.
- For the complex [Ni(PF₃)(CO)₃], since PF₃ is a better π-acceptor, it will compete with CO for back donation from the metal. This means that the extent of back donation to CO will be less than in the case of [Ni(CO)₄].

4. **Conclusion on Stretching Frequencies**:
- Since PF₃ competes with CO and is a better π-acceptor, the back donation to CO in [Ni(PF₃)(CO)₃] will be reduced compared to [Ni(CO)₄].
- This results in a lower bond order for the C-O bond in [Ni(PF₃)(CO)₃], leading to a lower stretching frequency compared to [Ni(CO)₄].

5. **Final Order of Stretching Frequencies**:
- Therefore, the order of stretching vibration frequency for the C-O bond is:
- [Ni(CO)₄] > [Ni(PF₃)(CO)₃]
- This can be represented as:
- I > II

### Answer:
The correct option is (b) I > II.