Boron is trivalent, we would expect a simple hydride . However, is not stable. Boron

possess incomplete octet and dimerises to form molecule and forms three centre 2-electron bond. The simplest boron hydride is diborane . As seen from the structure drawn, 6 electrons are required for the formation of conventional covalent bond structure by B -atom, whereas in diborane, there are 12 valence electrons; three from each boron atoms and six from the six hydrogen atoms. The geometry of can be represented as:

The four terminal hydrogen atoms and two boron atoms lie in one plane. Above and below the plane, there are two bridging hydrogen atoms. Each boron atom forms four bonds even though it has only three electrons. The terminal bonds are regular bonds but the bridge bonds are different.
Each bridge hydrogen is bonded to the two boron atoms only by sharing of two electrons. Such covalent bond is called 3-centre-2-electron bond or a multi centre bond or banana bond.