Assertion (A) : Low spin tetrahedral complexes are rarely observed.
Reason (R) : Crystal field splitting is less than pairing energy for tetrahedral complexes.

Options

A. Both A and R are true but R is the correct expanation of A
B. Both A and R are true but R is NOT the correct expanation of A
C. A is true but R is false
D. A is false and R is true

Explanation

To solve the question regarding the assertion and reason provided, we need to analyze both statements carefully.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that "Low spin tetrahedral complexes are rarely observed."
- This is a claim about the nature of tetrahedral complexes in coordination chemistry.

2. **Understanding the Reason (R)**:
- The reason states that "Crystal field splitting is less than pairing energy for tetrahedral complexes."
- This refers to the relationship between the energy required to pair electrons and the energy difference between the split d-orbitals in a tetrahedral field.

3. **Analyzing Tetrahedral Complexes**:
- In tetrahedral complexes, the d-orbitals split into two sets: the lower energy set (e) and the higher energy set (t2).
- The splitting energy (Δ_t) in tetrahedral complexes is generally smaller than that in octahedral complexes.

4. **Pairing Energy vs. Crystal Field Splitting**:
- Pairing energy is the energy required to pair two electrons in the same orbital.
- For tetrahedral complexes, the crystal field splitting (Δ_t) is less than the pairing energy. This means that it is energetically more favorable for electrons to occupy higher energy orbitals rather than pairing up in lower energy orbitals.

5. **Conclusion on Spin State**:
- Since the crystal field splitting is less than the pairing energy, electrons will occupy the higher energy orbitals (t2) before pairing occurs.
- Therefore, tetrahedral complexes tend to be high spin rather than low spin.

6. **Final Evaluation**:
- The assertion is true: Low spin tetrahedral complexes are rarely observed.
- The reason is also true: Crystal field splitting is indeed less than pairing energy for tetrahedral complexes.
- Since both the assertion and reason are true, and the reason correctly explains the assertion, we conclude that both statements are correct.

### Final Answer:
Both the assertion (A) and the reason (R) are true, and the reason correctly explains the assertion.

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