Question

What do you understand by electron deficient compounds of hydrogen?

Answer 1

Atoms or molecules having few no. of electrons than the required amount to attain maximum stability are called electron deficient species. These compounds do not have sufficient numbers of electrons to form normal covalent bonds.

Complete step by step solution:

Electron deficient compounds of hydrogen are hydrides of group 13, which has less number of electrons for usual covalent bonds. In order to make up this deficiency they exist in their polymeric forms. They are lewis acids, they can accept electrons.
Examples are $B{{H}_{3}},Al{{H}_{3}}$etc. They exist in polymeric form as ${{B}_{2}}{{H}_{6,}}{{B}_{4}}{{H}_{10}},{{(Al{{H}_{3}})}_{n}}$ etc.
Boranes are synthetic hydrides of boron with a formula${{B}_{x}}{{H}_{y}}$. We know that > Borane molecules are electron-deficient because they have multicenter bonding (having a pair of bonding electrons links more than two atoms, as in 3-center-2-electron bonds). This is to distinguish between these molecules from hydrocarbons from other classically bonded compounds. Some boranes, they are highly reactive depending on the electron-pair donors while others are not, for example the ${{B}_{n}}H_{n}^{2-}$dianions (n = 6-12) as well as many neutral boranes such as ${{B}_{18}}H_{22}^{{}}$. Some of the lower boranes are pyrophoric in air and react with water. The boranes belong to the class of cluster compounds, which have been the subject of developments in chemical bonding theory. Many of the related anionic hydridoborates are also synthesized.
Aluminium hydride (alane) is a white solid and may be tinted grey with decreasing particle size and impurity levels. Due to certain synthesis conditions, the surface of the alane could be passivated with a thin layer of aluminum oxide or hydroxide. Alane and its derivatives are used as reducing agents in many organic reactions.

Note: Electron deficient species are usually very reactive. Because of this reason the hydrides tend to form polymers of themselves.