Explain structures of diborane and boric acid.

Explanation

(a) Diborane
`B_(2)H_(6)` is an electron-deficient compound. `B_(2)H_(6)` has only 12 electrons – 6 `e^(–)` from 6 H atoms and 3 `e^(–)` each from 2 B atoms. Thus, after combining with 3 H atoms, none of the boron atoms has any electrons left. X-ray diffraction studies have shown the structure of diborane as:

2 boron and 4 terminal hydrogen atoms `(H_(t))` lie in one plane, while the other two bridging hydrogen atoms `(H_(b))` lie in a plane perpendicular to the plane of boron atoms. Again, of the two bridging hydrogen atoms, one H atom lies above the plane and the other lies below the plane. The terminal bonds are regular two-centre two-electron `(2c – 2e^(–))` bonds, while the two bridging (B–H–B) bonds are three-centre two-electron `(3c – 2e^(–))` bonds.

(b) Boric acid
Boric acid has a layered structure. Each planar `BO_(3)` unit is linked to one another through H atoms. The H atoms form a covalent bond with a `BO_(3)` unit, while a hydrogen bond is formed with another `BO_(3)` unit. In the given figure, the dotted lines represent hydrogen bonds.