Which of the following species are not known ?

Options

A. ` [SiCl_6 ]^(2-)`
B. `[CF_6]^(2-)`
C. `[PbCl_6]^(2-)`
D. `[SiF_6]^(2-)`

Explanation

To determine which of the following species are not known, we need to analyze the covalency and valency of the elements involved, particularly focusing on silicon and carbon.

### Step-by-Step Solution:

1. **Understanding Covalency and Valency:**
- Covalency refers to the number of bonds an atom can form, while valency is the measure of an atom's ability to combine with other atoms.
- Carbon typically has a maximum valency of 4, meaning it can form four bonds.

2. **Analyzing Silicon:**
- Silicon can expand its covalency due to the presence of vacant d orbitals. This allows silicon to form compounds with higher valency, such as 6.

3. **Evaluating Carbon Compounds:**
- Carbon, being tetravalent, cannot expand its covalency beyond 4 because it lacks vacant d orbitals. Therefore, compounds like CF6-2 are not theoretically possible.

4. **Considering Lead and Silicon:**
- Lead (Pb) and silicon (Si) can expand their covalency due to the availability of vacant orbitals. This means they can form compounds with higher valencies, such as 6.

5. **Conclusion:**
- Among the species mentioned, CF6-2 is not known because carbon cannot exceed its tetravalency of 4. Therefore, the correct answer is B (CF6-2).

### Final Answer:
The species that is not known is **B (CF6-2)**.

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