To determine the number of 2-centre-2-electron (2c-2e) and 3-centre-2-electron (3c-2e) bonds in diborane (B₂H₆), we need to understand its structure and bonding.

Structure of Diborane (B₂H₆):

Diborane has a unique structure with two boron atoms and six hydrogen atoms. It contains two types of hydrogen atoms:

• Terminal hydrogens: Four hydrogen atoms that are bonded normally to boron atoms.
• Bridge hydrogens: Two hydrogen atoms that form bridges between the two boron atoms.

Bonding in Diborane:

The bonding involves:

• 2c-2e bonds: These are conventional bonds where two atoms share two electrons. In B₂H₆, there are four B-H terminal bonds, each being a 2c-2e bond.
• 3c-2e bonds: These are bonds where three atoms (two boron and one hydrogen) share two electrons. In B₂₆, the two bridge hydrogens form two such bonds (each bridge H is part of one 3c-2e bond).

Counting the bonds:

• Number of 2c-2e bonds: 4 (all are B-H terminal bonds).
• Number of 3c-2e bonds: 2 (both are B-H-B bridge bonds).

Thus, the correct numbers are 4 and 2.

Related Topics:

• Electron-deficient compounds: Diborane is electron-deficient because it has fewer electrons than required for conventional bonds, leading to multi-center bonding.
• Boranes: Diborane is the simplest borane; higher boranes also exhibit 3c-2e bonds and other multi-center bonds.
• Hybridization: In diborane, boron atoms are approximately sp³ hybridized, but the bonding is not typical due to electron deficiency.

Formulae:

No specific formulae are needed for counting bonds, but understanding the electron count is important:

Total valence electrons in B₂H₆ = (2 × 3) + (6 × 1) = 12 electrons.

These are distributed as: 4 terminal B-H bonds use 8 electrons (4 bonds × 2e), and 2 bridge bonds use 4 electrons (2 bonds × 2e), totaling 12 electrons.