According to VSEPR model, molecules adopt geometries in which their valence electron pairs position themselves as far from each other as possible. The VSEPR model considers double and triple bonds to have slightly greater repulsive effects than single bonds because of the repulsive effect of `pi` -electrons. However the lone pair creates the maximum repulsive effect.
Which of the following does not represent the isostructural pair ?

Options

A. `SF_(5)^(-) and IF_(5)`
B. `ClO_(2)F_(3) and SOF_(4)`
C. `SeF_(3)^(+) and XeO_(3)`
D. None

Explanation

To determine which of the following does not represent an isostructural pair, we will analyze the given molecules based on their valence electron configurations, steric numbers, and hybridizations.

### Step-by-Step Solution:

1. **Identify the Molecules**:
- We have the following pairs to analyze:
- SF₆ and RIF₅
- ClO₂F₃ and SOF₄
- SeF₄ and XeO₃

2. **Analyze SF₆ and RIF₅**:
- **SF₆**:
- Sulfur (S) has 6 valence electrons (Group 16).
- It forms 6 bonds with fluorine (F), leading to 0 lone pairs.
- Steric number = 6 (6 bond pairs), Hybridization = sp³d².
- Geometry: Octahedral.
- **RIF₅**:
- Iodine (I) has 7 valence electrons (Group 17).
- It forms 5 bonds with fluorine and has 1 lone pair.
- Steric number = 6 (5 bond pairs + 1 lone pair), Hybridization = sp³d².
- Geometry: Distorted octahedral.
- **Conclusion**: Both have the same steric number and hybridization, thus they are isostructural.

3. **Analyze ClO₂F₃ and SOF₄**:
- **ClO₂F₃**:
- Chlorine (Cl) has 7 valence electrons.
- It forms 5 bonds (2 with O and 3 with F) and has 0 lone pairs.
- Steric number = 5, Hybridization = sp³d.
- Geometry: Trigonal bipyramidal.
- **SOF₄**:
- Sulfur (S) has 6 valence electrons.
- It forms 4 bonds (1 with O and 3 with F) and has 0 lone pairs.
- Steric number = 5, Hybridization = sp³d.
- Geometry: Trigonal bipyramidal.
- **Conclusion**: Both have the same steric number and hybridization, thus they are isostructural.

4. **Analyze SeF₄ and XeO₃**:
- **SeF₄**:
- Selenium (Se) has 6 valence electrons.
- It forms 4 bonds with fluorine and has 1 lone pair.
- Steric number = 5 (4 bond pairs + 1 lone pair), Hybridization = sp³d.
- Geometry: SeF₄ has a seesaw shape (due to the lone pair).
- **XeO₃**:
- Xenon (Xe) has 8 valence electrons.
- It forms 3 bonds with oxygen and has 1 lone pair.
- Steric number = 4 (3 bond pairs + 1 lone pair), Hybridization = sp³.
- Geometry: Bent or angular.
- **Conclusion**: They do not have the same steric number or hybridization, thus they are not isostructural.

### Final Answer:
The pair that does not represent an isostructural pair is **SeF₄ and XeO₃**.