The number of vacant hybrid orbitals which participate in the formation of 3-centre 2 electrons bonds i.e., banana bonds in diborane structure is :

Explanation

To solve the question regarding the number of vacant hybrid orbitals participating in the formation of 3-centre 2-electron bonds (banana bonds) in the structure of diborane (B2H6), we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Structure of Diborane**:
- Diborane (B2H6) consists of two boron atoms and six hydrogen atoms. Each boron atom has three valence electrons (electronic configuration: 1s² 2s² 2p¹).

2. **Determining Hybridization**:
- The boron atoms in diborane undergo hybridization to form sp³ hybrid orbitals. This means that one s orbital and three p orbitals combine to form four equivalent sp³ hybrid orbitals.

3. **Drawing the Structure**:
- In the diborane molecule, two boron atoms are connected to each other, and each boron atom is bonded to three hydrogen atoms. The structure can be visualized as:
- Two boron atoms at the center.
- Each boron atom forms two terminal bonds with hydrogen atoms and one bond that involves three atoms (the banana bond).

4. **Identifying 3-Centre 2-Electron Bonds**:
- The 3-centre 2-electron bonds (banana bonds) are formed between two boron atoms and one hydrogen atom. Each of these bonds involves the sharing of two electrons among three atoms.

5. **Counting Vacant Hybrid Orbitals**:
- In the diborane structure, each boron atom has four sp³ hybrid orbitals. However, two of these orbitals are used to form bonds with hydrogen atoms, and one orbital is used to form a 3-centre bond with the other boron and a hydrogen atom.
- This leaves one sp³ hybrid orbital on each boron atom vacant.

6. **Total Vacant Hybrid Orbitals**:
- Since there are two boron atoms in diborane, and each has one vacant hybrid orbital, the total number of vacant hybrid orbitals participating in the formation of banana bonds is:
- 1 (from the first boron) + 1 (from the second boron) = 2 vacant hybrid orbitals.

### Final Answer:
The number of vacant hybrid orbitals which participate in the formation of 3-centre 2-electron bonds (banana bonds) in diborane structure is **2**.

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