An electron deficient hydride does not have sufficient electrons to form a regular bond in which two electons are shared by atoms e.g. `B_(2)H_(6),Al_(2)H_(6)` etc. These hydrides cannot be represented by conventional Lewis structures. `B_(2)H_(6)` for example, contains four regular bonds and two three centered two electron bond. Its structures can be represented as:
Since these hydrides are electrons defiocient, they have a tendency to accept electrons. Hence, they are as Lewis acids.
`B_(2)H_(6)+2NMe to 2BH_(3).NMe`,
`B_(2)H_(6)+2CO to 2BH_(3).CO`