Question 1

What is a 3-centre-2-electron (3c-2e) bond?

Accepted Answer

It is a bond in which a single pair of electrons holds three atoms together. In diborane, one B sp³ orbital, one H 1s orbital and the other B sp³ orbital combine into three molecular orbitals; the two electrons fill the lowest (bonding) one, which spreads over B–H–B. Because this orbital is curved, it is often called a banana bond.

Question 2

Why can't diborane have an ethane-like structure?

Accepted Answer

B₂H₆ has only 12 valence electrons (2 × 3 from boron + 6 × 1 from hydrogen). An ethane-like H₃B–BH₃ structure would need 7 two-electron bonds, i.e. 14 electrons. With two electrons short, the molecule instead forms four normal terminal B–H bonds and two 3c-2e B–H–B bridges.

Question 3

Why are the bridging B–H bonds longer than the terminal ones?

Accepted Answer

Each terminal B–H bond has a full electron pair to itself, while each bridging electron pair is shared between two B–H links. That gives each bridging B–H link a bond order of about ½, so it is weaker and longer — about 133 pm compared with about 119 pm for the terminal bonds.

Question 4

Is the bridge in Al₂Cl₆ also a 3c-2e bond?

Accepted Answer

No. In Al₂Cl₆ the bridging chlorine has lone pairs, and it donates one to the second aluminium as a dative bond. Each Al–Cl–Al bridge therefore holds four electrons — a 3c-4e bond. Genuine 3c-2e bridges appear when the bridging group has no lone pair, as in B₂H₆, Al₂(CH₃)₆ and polymeric BeH₂.

Question 5

What is the hybridisation of boron in diborane?

Accepted Answer

Each boron is sp³ hybridised. Two of its sp³ orbitals form the terminal B–H bonds and the other two take part in the two B–H–B bridges, giving a roughly tetrahedral arrangement around each boron.

3c-2e Bond — Why Diborane Needs Banana Bonds

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