Many boron hydrides, BnHm, are known, and the most important is diborane, B2H6.
All are endothermic in formation, and have a formal boron oxidation state of less than three: some contain B-B bonds.
They are reducing, acting by hydride (H–) ion transfer.
|Structure: Each B atom is tetrahedrally coordinated by H atoms. The central B-H-B bonds (the bridging bonds) are 3-centered, 2-electron [3c,2e] bonds.|
The [3c,2e] bonds in diborane (one for each of the B-H-B bridging groups) can be pictured as two 1-electron bonds. Usually, two electrons are required for each covalent bond, but here the wave functions of the two electrons span orbitals from all three atoms, with some delocalization of character, rather like the bonding in benzene, where the wave function of each of the electrons in the p-system spans all six of the carbon atoms.
Reactions of Diborane: