Diborane : The four terminal hydrogen atoms and the two boron atoms lie in one plane. Above and below this plane, there are two bridging hydrogen atoms. The four terminal B – H bonds are regular two centre-two electron bonds (2c – 2e) while the two bridge (B – H – B) bonds are different and can be described in terms of three centre-two electron bonds (3c – 2e). Each B atom uses $sp^3$ hybrids for bonding. Out of the four $sp^3$ hybrids on each B atom, one is without an electron shown in broken lines. The terminal B-H bonds are normal 2-centre-2-electron bonds but the two bridge bonds are 3-centre-2-electron bonds. The 3-centre-2-electron bridge bonds are also referred to as banana bonds. Boric acid : In boric acid, planar $BO_3$ units are joined by hydrogen bonds to give a layered structure.