What do you understand by (i) electron-deficient, (ii) electron-precise and (iii) electron-rich compounds of hydrogen? Provide justification with suitable examples.

Explanation

Hydrides of group 13 (i.e, `BH_(3)` , `AlH_(3)` etc.) donot have sufficient number of electrons to form normal covalant bonds and hence are called elctron-deficient hydrides. To make up this deficieny ,they generally exit in polymeric forms such as `B_(2)H_(6)`, `B_(4)H_(10)` , `(AlH_(3))` etc.
(ii) Hydrides of gropu 14 (i.e, `CH_(4)` , `SiH_(4)`, `GeH_(4)`, `SnH_(4)`, `PbH_(4)`) have exact number of electrons to form covalent bonds and hence are called electron-precise hydrides. All these hydrides have tetrahedral shapes.
(iii) Hydrides of group 15,16 and 17 (i.e, `NH_(3)` ,`PH_(3)`,`H_(2)O`,`H_(2)S`,HF,HCl, etc.) have more electrons than required to form normal covalent bonds and hence are called electron-rich hydrides. The excess electrons in these hydrides are present as lone pairs of electrons.