An electron-deficient hydride does not have sufficient electrons to form a regular bond in which two electrons are shared by two atoms e.g., B2H6, Al2H6 etc. These hydrides cannot be represented by conventional Lewis structures. B2H6, for example, contains four regular bonds and two three centered-two electron bond. Its structure can be represented as:
Since these hydrides are electron-deficient, they have a tendency to accept electrons. Hence, they act as Lewis acids.