Question

The structure of diborane (B₂H₆ ) contains
(a) four 2c-2e bonds and two 3c-2e bonds
(b) two 2c-2e bonds and four 3c-2e bonds
(c) two 2c-2e bonds and two 3c-3e bonds
(d) four 2c-2e bonds and four 3c-2e bonds

Answer 1

(a) : According to molecular orbital theory, each of the two boron atoms is in sp³ hybrid state. Of the four hybrid orbitals, three have one electron each while the fourth is empty. Two of the four orbitals of each of the boron atom overlap with two terminal hydrogen atoms forming two normal B – H σ-bonds. One of the remaining hybrid orbital (either filled or empty) of one of the boron atoms, 1s orbital of hydrogen atoms (bridge atom) and one of hybrid orbitals of the other boron atom overlap to form a delocalised orbital covering the three nuclei with a pair of electrons. Such a bond is known as three centre two electron (3c – 2e) bonds.