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

Options

A. `CO_(3)^(2-) and NO_(3)^(-)`
B. `PCl_(4)^(+) and SiCl_(4)`
C. `PF_(5) and BrF_(5)`
D. `AlF_(6)^(3-) and SF_(6)`

Answer

To determine which of the given pairs of species are not isostructural, we need to analyze the molecular geometry and hybridization of each species in the pairs. Isostructural species have the same geometry and bond angles. Let's go through each pair step by step.

### Step 1: Analyze Pair 1 - Carbonate (CO3^2-) and Nitrate (NO3^-)

1. **Carbonate (CO3^2-)**:
- Structure: The carbonate ion has a central carbon atom bonded to three oxygen atoms. It has one double bond and two single bonds, with resonance structures contributing to the overall structure.
- Geometry: Trigonal planar.
- Bond Angles: Approximately 120 degrees.

2. **Nitrate (NO3^-)**:
- Structure: The nitrate ion has a central nitrogen atom bonded to three oxygen atoms, similar to carbonate, with one double bond and two single bonds.
- Geometry: Trigonal planar.
- Bond Angles: Approximately 120 degrees.

**Conclusion**: Both are trigonal planar and have the same bond angles, so they are isostructural.

### Step 2: Analyze Pair 2 - PCl4^+ and SiCl4

1. **PCl4^+**:
- Structure: The phosphorus atom is bonded to four chlorine atoms.
- Geometry: Tetrahedral.
- Hybridization: sp^3.

2. **SiCl4**:
- Structure: The silicon atom is also bonded to four chlorine atoms.
- Geometry: Tetrahedral.
- Hybridization: sp^3.

**Conclusion**: Both are tetrahedral and have the same hybridization, so they are isostructural.

### Step 3: Analyze Pair 3 - PF5 and BrF5

1. **PF5**:
- Structure: The phosphorus atom is bonded to five fluorine atoms.
- Geometry: Trigonal bipyramidal.
- Hybridization: sp^3d.

2. **BrF5**:
- Structure: The bromine atom is bonded to five fluorine atoms, but it has a lone pair of electrons.
- Geometry: Square pyramidal.
- Hybridization: sp^3d^2.

**Conclusion**: PF5 is trigonal bipyramidal while BrF5 is square pyramidal, so they are not isostructural.

### Step 4: Analyze Pair 4 - AlF6^3- and SF6

1. **AlF6^3-**:
- Structure: The aluminum atom is bonded to six fluorine atoms.
- Geometry: Octahedral.
- Hybridization: sp^3d^2.

2. **SF6**:
- Structure: The sulfur atom is also bonded to six fluorine atoms.
- Geometry: Octahedral.
- Hybridization: sp^3d^2.

**Conclusion**: Both are octahedral and have the same hybridization, so they are isostructural.

### Final Conclusion

From the analysis, we find that:
- Pair 1 (CO3^2- and NO3^-) is isostructural.
- Pair 2 (PCl4^+ and SiCl4) is isostructural.
- Pair 3 (PF5 and BrF5) is **not** isostructural.
- Pair 4 (AlF6^3- and SF6) is isostructural.

Thus, the answer is that **Pair 3 (PF5 and BrF5) are not isostructural**.