Which of the following is not known ?

Options

A. `SF_6`
B. `SCl_6`
C. `SF_4`
D. `SCl_4`

Explanation

To determine which of the following compounds is not known, we will analyze each option provided in the question: SF6, SCl6, SF4, and SCl4.

### Step-by-Step Solution:

1. **Identify the Compounds:**
- The compounds in question are:
- SF6 (Sulfur hexafluoride)
- SCl6 (Sulfur hexachloride)
- SF4 (Sulfur tetrafluoride)
- SCl4 (Sulfur tetrachloride)

2. **Analyze SF6:**
- SF6 is known to exist. It has a stable octahedral structure due to the ability of sulfur to accommodate six fluorine atoms, which are small and highly electronegative.

3. **Analyze SCl6:**
- SCl6 is not known to exist. Chlorine is larger than fluorine, and its lower electronegativity combined with its larger size means that six chlorine atoms cannot be accommodated around a single sulfur atom.

4. **Analyze SF4:**
- SF4 is known to exist. It has a seesaw shape due to the presence of four fluorine atoms around the sulfur atom, which allows for a stable structure.

5. **Analyze SCl4:**
- SCl4 is also known to exist. Similar to SF4, it has a stable structure with four chlorine atoms around the sulfur atom, which can accommodate them without issue.

6. **Conclusion:**
- Among the compounds analyzed, SCl6 is the only compound that is not known to exist. Therefore, the correct answer is SCl6.

### Final Answer:
**SCl6 is not known.**