Structure of Diborane

Diborane also known by the name of boron ethane, boron hydride, and diboron hexahydride is chemically represented as B2H6. Diborane is the main member of the family of boron compounds. The molar mass of diborane is 27.66 grams/moles, which is the sum of all constituent elements present in diborane (B2H6).

Bonding in diborane

As the name suggests, diborane is a compound of two atoms of boron, and with these two atoms of boron, six atoms of hydrogen are also present. As we know that the atomic number of boron is 5, and its electronic configuration is written as 1s2, 2s22p1, from the configuration we see that in the outermost shell of the boron three valence electrons are present. These three valence electrons take part in the formation of bonds with the hydrogen. 

Out of the three valence electrons of each boron atom, two electrons make bonds with the two hydrogen atoms by sharing electrons as a result of a formation of the covalent bond. This type of bond is known as two centre–two electron bond. The formation of this covalent bond is justified by the molecular orbital theory. The two hydrogen atoms are called the terminal hydrogen as they are present at the terminal end of the molecule.

The one remaining electron of the boron makes the bridge bond with one hydrogen as the three centre–two electron bond, where the three centres are boron–hydrogen–boron. In this bond, two electrons are involved. This bridge bond is also known as the banana bond as its structure is somehow bent like a banana and the character of this bond shows ionic nature.

The length of the bond between the terminal hydrogen and boron atom in the structure of diborane is 1.19 Å. The length of the bond between the bridge hydrogen and boron atom in the structure of diborane is 1.33 Å. The length of the terminal boron hydrogen bond is less than the bridge boron hydrogen bond, so it shows greater strength. 

The value of the frequency in the infrared spectrum is more compared with the bridge boron hydrogen bond. A stronger bond vibrates with more energy and shows high vibrational frequency. So, the frequency of the terminal boron hydrogen should be more than the bridge boron hydrogen bond. The value of the frequencies of the bridge Boron–Hydrogen bond and the terminal Boron–Hydrogen bond is 2100 and 2500 cm−1, respectively.                             

Structure of diborane

From the above description of bonding in diborane, the structure of diborane is shown as:                                         

In the structure of diborane, each boron atom is present in sp3 hybridization, which means that boron atoms have four hybrid orbitals, which take part in the formation of bonds. Out of these four hybrid orbitals, only three orbitals can carry electrons to make bonds. The two hybrid orbitals make boron–hydrogen bonds and one hybrid orbital make boron–hydrogen bridge bond. Out of these four hybrid orbitals, one of the hybrid orbitals remains vacant and participates in the formation of another banana bond with the bridge hydrogen. 

The structure of diborane is non-planar as four terminal hydrogens and two boron atoms are present in one plane and the bridge hydrogens are present above and below that plane to the perpendicular plane. These diborane molecules come under the category of D2h point group symmetry, which means that the structure of diborane consists of the identity operation, inversion through the centre of symmetry and three C2 rotational axes along the three sigma bonds between the boron atom and hydrogen atom.

Conclusion

The structure of diborane is somewhat different compared with other organic molecules in chemistry because the diborane molecule consists of two B–H–B banana bonds as a bridge bond in addition to four terminal normal B–H covalent bonds. The structure of diborane is non-planar as bridge hydrogens are present below and above another plane.