Banana Bond is an extended covalent bond found in boranes in which a hydrogen atom is shared by two boron atoms and form of three-center two-electron bond.
A banana bond is also known as a bent bond. It is a type of chemical bonding where the ordinary hybridization state of two atoms making up a chemical bond are modified with increased or decreased s-orbital character in order to accommodate a particular molecular geometry (that of 3 bananas in a ring shape).
Certain atoms, such as oxygen, due to their electron configuration will almost always set their two (or more) covalent bonds in non-collinear directions. H2O is an example of a bent molecule. The bond angle between the two hydrogen atoms is approximately 104.45°.
Let us take example: formation of Banana Bond in Borane
In B2H6 there are 12 valence electrons (B has 3, H has one hence 2B+6H=3*2+6*1=12)
out of 6H, 4H are bonded covalently with the two 2 B hence 8 electrons are used there(2 electrons are shared with 2 atoms).
The remaining 4 electrons are shared between the two bridging H atoms and the two B atoms consequently two bridging B-H-B bonds are formed each of which consists of 2 electrons( 2 electrons are shared with 3 atoms) which is sometimes called 'banana bond', as they are not linear but curved.