Which of the following methyl diboranes does not exist ?

Options

A. `B_(2)H_(4)(CH_(3))_(2)`
B. `B_(2)H_(5)(CH_(3))`
C. `B_(2)H_(2)(CH_(3))_(4)`
D. `B_(2)H(CH_(3))_(5)`

Answer

To determine which of the following methyl diboranes does not exist, we need to analyze the possible structures of methyl diboranes derived from borane (B2H6) and the methyl group (CH3).

### Step-by-Step Solution:

1. **Understanding Borane**:
- Borane is a compound with the formula B2H6. It consists of two boron atoms and six hydrogen atoms.

2. **Formation of Methyl Diboranes**:
- Methyl diboranes are formed when borane reacts with methyl groups (CH3). The general formula for methyl diboranes can be represented as B2H6-x(CH3)x, where x is the number of methyl groups replacing hydrogen atoms.

3. **Possible Structures**:
- We can derive several possible structures for methyl diboranes by substituting hydrogen atoms in borane with methyl groups:
- **Option 1**: B2H2(CH3)4
- **Option 2**: B2H3(CH3)3
- **Option 3**: B2H4(CH3)2
- **Option 4**: B2H5(CH3)

4. **Counting Methyl Groups**:
- In the first structure (Option 1), there are 4 methyl groups.
- In the second structure (Option 2), there are 3 methyl groups.
- In the third structure (Option 3), there are 2 methyl groups.
- In the fourth structure (Option 4), there is 1 methyl group.

5. **Analyzing Steric Hindrance**:
- The last structure (Option 4: B2H5(CH3)) suggests that there are 5 methyl groups attached to the boron atoms. This creates significant steric hindrance because boron can only accommodate a limited number of bulky groups due to its geometry and electron deficiency.

6. **Conclusion**:
- Given the steric hindrance and the inability of boron to stabilize five bulky methyl groups, the structure B2H5(CH3) cannot exist. Therefore, the answer to the question is **Option 4**.

### Final Answer:
**Option 4: B2H5(CH3) does not exist.**