$(a)$ Diborane is an electron deficient compound in which only $12$ valence electrons are present. $2 B$ atoms and $4$ terminal $H$ atoms are co-planar whereas two bridging hydrogen are present in perpendicular plane. One bridging hydrogen is above the plane and other bridging $H$ atom is below the plane. Terminal $B – H$ bonds are $2 c -2 e$ bonds and bridging $B – H – B$ bonds are $3 c$ $-2 e$ bonds.

$(b)$ Boric acid has layered structure. Planar $BO _3$ unit is linked to another unit through $H$ atoms. The $H$ atoms form covalent bond with $BO _3$ unit and hydrogen bond with other $BO _3$ unit.