Molecular hydrides are classified according to the relative numbers of electrons and bonds in Lewis structure as follow
(i) Electron deficient hydrides These types of hydrides contain central atom with incomplete octet. These are formed by 13 group elements, e.g., BH3, AlH3, etc. To complete their octet they generally exist in polymeric forms such as B2H6, B4H10, (AlH3)n etc. These hydrides act as Lewis acids.
(ii) Electron precise hydrides These hydrides have exact number of electrons required to form normal covalent bonds. These are formed by 14 group elements, e.g., CH4, SiH4, etc. These are tetrahedral in shape.
(iii) Electron rich hydrides These hydrides contain central atom with excess electrons, which are present as lone pairs.
These are formed by 15, 16 and 17 group elements, e.g., NH3, H2O, HF, etc. These hydrides act as Lewis bases.
