(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 – 2$e^{-}$ ) bonds, while the two bridging (B–H–B) bonds are three-centre two-electron (3c – 2$e^{-}$ ) bonds.

(b) Boric acid
Boric acid has a layered structure. Each planar $B{O}_3$ unit is linked to one another through H atoms. The H atoms form a covalent bond with a $B{O}_3$ unit, while a hydrogen bond is formed with another $B{O}_3$ unit. In the given figure, the dotted lines represent hydrogen bonds.