HYDRIDES

The hydrides are classified into three types:
(i) Ionic or saline or salt like hydrides
(ii) Covalent or molecular hydrides (iii) Metallic or non-stoichiometric hydrides.

• Ionic or Saline Hydrides
Hydrides formed between hydrogen and electropositive element of group I and II belonging to s-block. These are known as stoichiometric compounds.
Properties of saline or ionic hydrides:
(i) The hydrides of lighter elements like Li, Be, Mg etc. have significant covalent character.
(ii) Ionic hydrides are crystalline, non-volatile and non-conducting in solid state.
(iii) They conduct electricity in molten state and liberate hydrogen at anode.

• Covalent or Molecular Hydrides
These are binary compounds of hydrogen with non-metals belonging to p-block.
For example, NH3, CH4, H20, HF They are mostly volatile compounds with low boiling points. They are classified as:
(i) Electron-Deficient Molecular Hydride: Molecular hydrides in which central atom does not have octet are called electron deficient hydrides e.g., BH3, MgH2, BeH2.
(ii) Electron precise hydrides: Those hydrides in which the central atom has its octet complete e.g., group 14 hydrides. They are tetrahedral in geometry.
(iii) Electron rich hydrides: Those metal hydrides which contain lone pair of electrons are called electron rich hydrides, e.g., NH3, PH3, H20 and H2S.
NH3 and PH3 has 1 lone pair and H20 and H2S have 2 lone pairs of electrons.

• Metallic or Non-Stoichiometric Hydrides
These hydrides are also known as interstitial hydrides. Transition metals group 3, 4 and 5 form metallic hydrides. In group 6, chromium alone has a tendency to form CrH. Metals of 7, 8 and 9 do not form hydrides. This is called as hydride gap.
Latest study shows that only Ni, Pd, Ce and Ac are interstitial in nature, that means they can occupy hydrogen atom in the interstitial sides. The hydrides are generally non-stoichiometric and their composition varies with temperature and pressure, for example, Ti H1.73, CeH2.7′ , LaH2.8  etc.
These hydrides have metallic lock and their properties are closely related to those of the parent metal. They are strong reducing agents in most of the cases due to the presence of free hydrogen atom in the metal lattice.