THE THREE CENTERED TWO ELECTRON BOND

A three centered two electron bond as name suggest speaks about three centres sharing two electrons to make one  bond.
These type of bonding is possible only in electron deficient species
eg:- bh3 dimerises  to form b2h6.

The stability of  B2H6 is well established.

In the above structure a,b,c,d represent conventional 2 centered two electron bond ,but e and f represent three center 2 electron bond .

B2H6 has 12 electron out of which 8 electron are used in forming B-H bonds and remaining 4 electron is used in 3 center 2 electron bond.Thus we have two B-H-B bridging structure.

let us understand the stability of these structure considering mot.

So we consider group orbitals of BH3 system.

If we consider BH3  it has six electrons thus the d orbital would be the one which is vacant.

So we need to mix d orbitals which are basically pure p orbital .

So now we have  P+P AND P-P ORBITAL .

THESE orbital mix with the 1s orbital of hydrogen and give us P+P-S ,P-P+S,P+P+S.