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

Electron deficient compounds. Hydrides of group 13 (i.e. BH₃, AlH₃etc.) have lesser electrons to form normal covalent bonds and hence are called electron deficient hydrides. To make up this deficiency, these hydrides generally exist in polymeric forms such as B₂H₆, B₄H₁₀, (AlH₃)_n etc. They act as Lewis acids i.e. electron acceptors.
(ii) Electron-precise compounds. Electron precise compounds have the required number of electrons to write their conventional Lewis structures. All elements of group 14 form such compounds (i.e. CH₄, SiH₄, GeH₄, SnH₄, PbH₄), which are tetrahedral in geometry. They do not act as Lewis acids or Lewis bases.
(iii) Electron rich compounds. Electron rich hydrides have excess electrons which are present as lone pairs. Elements of group 15, 16, 17 form such compounds (NH₃, PH₃, H₂O, H₂S, HF, HCl etc.). They all behave as Lewis bases i.e. electron donors.