Question

Which of the following statements is incorrect with respect to the structure of diborane?

• A. All terminal $B-H$ bond lengths are equal.
• B. The terminal $B-H$ bond is a $2$-centre $3$- electron bond.
• C. The terminal $B-H$ bond is a $2$-centre $2$- electron bond.
• D. The bridge $B-H-B$ is a $3$-centre $2$-electron bond.

Answer 1

The correct option is B The terminal $B-H$ bond is a $2$-centre $3$- electron bond.

The model of diborane indicates that the bonds between boron and the terminal hydrogen atoms are conventional $2$-centre $2$-electron covalent bonds. The bonding between the boron atoms and the bridging hydrogen atoms is however, different from that in molecules such as hydrocarbons. Having used two electrons in bonding to the terminal hydrogen atoms, each boron has one valence electron remaining for additional bonding. The bridging hydrogen atoms provide one electron each. Thus, the $B_2{H}_2$ ring is held together by four electrons, which is an example of $3$-centre $2$-electron bonding.