Orbital structure of Diborane :
Diborane has less number of electrons than required for the formation of normal bonds between the atoms. Thus, diborane is an Electron deficient molecule.
Diborane formula is 
. It is a colourless gas.
Diborane structure :
In diborane, each Boron atom provides three electrons and each hydrogen atom provides one electron for bonding. It has 12 valency electrons in total which can form six bonds only.
In diborane each boron atom in an excited state undergoes 
hybridization resulting in the formation of four – hybrid orbitals. Three of them has a single electron each and the fourth one is vacant. At each boron atom two hybrid orbitals with single electron are overlapped by the s – orbital of H – atom forming two B – H covalent bonds. Thus in the molecule of diborane, there are four B – H covalent bonds which are planar (Terminal).
The
hybrid orbitals of one boron atom with an unpaired electron, and the
hybrid orbital of another boron atom which is vacant is overlapped by s – orbital of the hydrogen atom. Thus a pair of electrons is linking three atoms (two Boron and one H – atom) by delocalization. In this way, two bonds are formed as shown. This kind of bond is called three centered two electron bond. This is taking curvature due to repulsions between nuclei of bridge hydrogen bonds, hence is also called
banana bond. The two bridge hydrogen atoms are perpendicular to the planar hydrogen atom as shown :
