Hydrides

Hydride is a chemical compound where the hydrogen atoms release nucleophilic, basic or reducing properties; in simple terms, you can also say that it is an anion of hydrogen. Hydrides (ExHy) are named after binary compounds that hydrogen creates with other characters of the periodic table.

The hydrogen atom is bonded to a more electropositive element or group in compounds that are said to be hydrides. But, hydrides react vigorously with water, usually irreversible and all this happens because of the basic metal-hydrogen bond polarity.

Types of Hydrides

Here are the three types of hydrides:

• Saline or Ionic Hydride
• Metallic Hydride
• Covalent Hydride

The type of chemical bond involved is what makes them different from each other. One more kind of hydride called dimeric hydride which is recognized based on its structure.

Let us know about all the types of hydrides in detail-

a)    Ionic Hydrides-

They are also called saline hydrides or pseudohalides. The existence of hydrogen identifies these hydrides as a negatively charged ion, H-. They are created when a hydrogen molecule reacts with s-block elements that are highly electropositive. In the periodic table, ionic hydrides are the most active elements.

Ionic compounds form between hydrogen and the most active metals, usually with alkali and alkaline-earth metals of groups one and two elements.

 The hydride ion reacts with water, consequently NaH and CaH2 and is usually used as drying agents in various industries, unlike the halide ions, which are soluble in water. 

 These metals directly react with hydrogen at high temperatures, creating hydrides of general formulas MH and MH2. Beryllium and Magnesium are alkaline earth metals that also form stoichiometric MH2 hydrides. But, both these hydrides are relatively covalent by their characteristics.

The typical ionic hydrides are Group 1 (IA) metals and the heavier group 2 (IIA) metals. It is essential for you to know ionic hydrides behave distinctively in a solid and liquid state.

They are crystalline, non-volatile and non-conducting when in a solid-state; conversely, they act as good conductors of electricity in the liquid form. Hydrogen gas is generated at the anode in electrolysis.

 MH(s) + H2O(l) → MOH(aq) + H2(g)

 Ionic hydrides are insoluble in regular solvents; they present their non-molecular structures. Usually, they exist as binary materials that comprise two elements: hydrogen. They are mainly used as heterogeneous bases and in organic synthesis as reducing agents.

b)    Metallic Hydrides-

They are created by heating hydrogen gas with metals or its alloys. In simple terms, when a hydrogen compound structures a bond with any metal element, that is stated to be a metal hydride. The formed bond is usually covalent, but the hydrides are sometimes created with ionic bonds.

 There is one unique feature about metal hydrides that you must know. They can be nonstoichiometric; this means that the fraction of hydrogen atoms to the metals is not steady; they have a static composition. The reason is that metal and hydrogen bonding is a crystal mesh that hydrogen items may fill in between the mesh and is not a fixed order filling.

 Therefore, the ratio between hydrogen atoms and metals is not fixed, although metallic hydrides comprise basic stoichiometric compounds. Metallic hydrides are best-considered alloys because they do not have definite stoichiometric compounds.

 These hydrides are very hard in nature as they have high boiling and melting points. You should keep in mind that all d and f block elements are transition elements; they are different compared to ionic hydrides that can conduct electricity.

 Some examples of metallic hydrides are Magnesium hydride, aluminium hydride and cadmium hydride.

c)    Covalent Hydrides- 

These hydrides comprise all other compounds that possess hydrogen. They are the primary compounds of hydrogen and nonmetals; the bonds are undoubtedly electron pairs shared by atoms of comparable electronegativities.

 They are created when hydrogen reacts with similar electronegative elements such as C, Si, etc. It shares a covalent bond which can be either a volatile or non-volatile compound. Volatile means anything that can be quickly vaporized at low temperatures.

 They can be in both liquid or gas form, with low melting and boiling points except where hydrogen bonding changes their properties.

 Magnesium hydride acts as an intermediate between covalent and ionic because they have a polymeric solid, which is just like AIH3, but it reacts quickly with ionic hydrides like water. 

Covalent bonds are usually created by elements in the groups 13 to 17-

Boron, aluminium and gallium of group 13 together can form covalent hydrides. The hydrides from this group are usually less efficient at releasing an electron; they keep them because their electron orbital becomes fuller. This means that rather than donating H-, it would donate H+ as they are relatively more acidic.

A few definitions restrict hydrides to hydrogen centres that formally react as hydrides, nucleophiles and hydrogen atoms bound to metal centres. Some examples of Covalent hydrides are Silane, boron hydrides, nitrogen hydrides, etc.

They are made by a series of synthetic routes. The easiest is the direct mix of the elements. 

Conclusion 

This was about hydrides and their different types, used in various industries and fields for specific purposes. Some of the most well-known examples of Hydrides are water, methane and ammonia. Hydrogen is an important topic students should study the topic carefully.