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

Options

A. `Co_3^(2-)` and `NO_3^-`
B. `Pcl_4^+` and `SlCl_4`
C. `PF_5` and `BrF_6`
D. `AIF_6^(3-)` and `SF_6`

Answer

To determine which of the following pairs of species are not isostructural, we need to analyze the electron count, hybridization, and resulting molecular geometry for each species in the pairs. Isostructural species have the same geometry and hybridization.

### Step-by-Step Solution:

1. **Identify the Electron Count and Hybridization for Each Species:**
- For each species, calculate the total number of valence electrons by considering the contribution from each atom and any charges present.

2. **Determine the Hybridization and Geometry:**
- Use the electron count to determine the hybridization:
- 4 electrons: sp (linear)
- 6 electrons: sp² (trigonal planar)
- 8 electrons: sp³ (tetrahedral)
- 10 electrons: sp³d (trigonal bipyramidal)
- 12 electrons: sp³d² (octahedral)

3. **Analyze Each Pair:**
- **Pair 1: CO₃²⁻ and NO₃⁻**
- CO₃²⁻: Carbon (4) + 3 Oxygen (6) + 2 (charge) = 12 electrons → sp³d² (octahedral)
- NO₃⁻: Nitrogen (5) + 3 Oxygen (6) + 1 (charge) = 12 electrons → sp³d² (octahedral)
- Both are isostructural (octahedral).

- **Pair 2: PCl₄⁺ and SiCl₄**
- PCl₄⁺: Phosphorus (5) + 4 Chlorine (4) - 1 (charge) = 8 electrons → sp³ (tetrahedral)
- SiCl₄: Silicon (4) + 4 Chlorine (4) = 8 electrons → sp³ (tetrahedral)
- Both are isostructural (tetrahedral).

- **Pair 3: PF₅ and BrF₆**
- PF₅: Phosphorus (5) + 5 Fluorine (5) = 10 electrons → sp³d (trigonal bipyramidal)
- BrF₆: Bromine (7) + 6 Fluorine (6) = 13 electrons → sp³d² (octahedral)
- Not isostructural (different geometries).

- **Pair 4: AlF₆³⁻ and SF₆**
- AlF₆³⁻: Aluminum (3) + 6 Fluorine (6) + 3 (charge) = 12 electrons → sp³d² (octahedral)
- SF₆: Sulfur (6) + 6 Fluorine (6) = 12 electrons → sp³d² (octahedral)
- Both are isostructural (octahedral).

4. **Conclusion:**
- The pair that is not isostructural is **PF₅ and BrF₆**.

### Final Answer:
**The pair that is not isostructural is PF₅ and BrF₆.**