In which of the following pairs, the two species are not isostructural?

Options

A. `CO_(3)^(2+)` and `NO_(3)^(-)`
B. `PCI_(4)^(+)` and `SiSI_(4)`
C. `PF_(5)` and `BrF_(5)`
D. `AIF_(6)^(3-)` and `SF_(6)`

Explanation

To determine which of the given pairs of species are not isostructural, we need to analyze the molecular geometries of each pair based on their Lewis structures and hybridization.

### Step-by-Step Solution:

1. **Identify the pairs**: The pairs given in the question are:
- A: \( \text{CO}_3^{2-} \) and \( \text{NO}_3^{-} \)
- B: \( \text{PCl}_4^{+} \) and \( \text{SiCl}_4 \)
- C: \( \text{PF}_5 \) and \( \text{BrF}_5 \)
- D: \( \text{AlF}_6^{3-} \) and \( \text{SF}_6 \)

2. **Analyze Pair A: \( \text{CO}_3^{2-} \) and \( \text{NO}_3^{-} \)**:
- **\( \text{CO}_3^{2-} \)**: The carbon atom is bonded to three oxygen atoms, with one double bond and two single bonds. The hybridization is \( sp^2 \), and the geometry is trigonal planar.
- **\( \text{NO}_3^{-} \)**: The nitrogen atom is bonded to three oxygen atoms, similar to carbonate, with one double bond and two single bonds. The hybridization is also \( sp^2 \), and the geometry is trigonal planar.
- **Conclusion**: Both are isostructural.

3. **Analyze Pair B: \( \text{PCl}_4^{+} \) and \( \text{SiCl}_4 \)**:
- **\( \text{PCl}_4^{+} \)**: The phosphorus atom has four chlorine atoms bonded to it. The hybridization is \( sp^3 \), and the geometry is tetrahedral.
- **\( \text{SiCl}_4 \)**: The silicon atom also has four chlorine atoms bonded to it. The hybridization is also \( sp^3 \), and the geometry is tetrahedral.
- **Conclusion**: Both are isostructural.

4. **Analyze Pair C: \( \text{PF}_5 \) and \( \text{BrF}_5 \)**:
- **\( \text{PF}_5 \)**: The phosphorus atom is bonded to five fluorine atoms. The hybridization is \( sp^3d \), and the geometry is trigonal bipyramidal.
- **\( \text{BrF}_5 \)**: The bromine atom is bonded to five fluorine atoms and has one lone pair. The hybridization is \( sp^3d^2 \), and the geometry is square pyramidal.
- **Conclusion**: These are not isostructural due to different geometries.

5. **Analyze Pair D: \( \text{AlF}_6^{3-} \) and \( \text{SF}_6 \)**:
- **\( \text{AlF}_6^{3-} \)**: The aluminum atom is bonded to six fluorine atoms. The hybridization is \( sp^3d^2 \), and the geometry is octahedral.
- **\( \text{SF}_6 \)**: The sulfur atom is also bonded to six fluorine atoms. The hybridization is \( sp^3d^2 \), and the geometry is octahedral.
- **Conclusion**: Both are isostructural.

### Final Answer:
The pair that is not isostructural is **C: \( \text{PF}_5 \) and \( \text{BrF}_5 \)**.