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What is covalent hydride?

Before going into the discussion about covalent hydride we will first see a bit about the hydride and its types.

Hydride

A hydride is nothing but a compound where hydrogen is attached to a less electronegative element. In this particular, chemical compound the atoms of hydrogen exhibit properties that are basic, nucleophilic, or reducing in nature. Generally, in a hydride, the oxidation number of hydrogen is equal to −1. So, in simple terms compounds that have covalently bonded Hydrogen atoms are known as hydrides. Some examples are ammonia where H atoms are bonded to nitrogen(NH3), water which is a hydride of oxygen(H2O), methane (CH4) which is a hydride of carbon, and so on. A hydride is formed when hydrogen gets attached to a less electronegative substance (element). Almost all the elements form binary compounds with hydrogen.

Some of the properties of a hydride are highlighted below:

IUPAC NameHydride
Molecular FormulaH
Molecular Weight1.008 g/mol
Chemical NameHydrogen Anion

The properties of the formed hydride depend on various things like the type of intermolecular force existing between hydrogen and other elements, the molecular masses, temperature, and many other factors.

Uses of Hydrides

  • Many hydrides are commonly used as reducing agents required especially in the synthesis of chemicals. Some of which include lithium aluminium hydride, sodium borohydride, DIBAL (di isobutyl aluminium hydride) and, super hydride. Here the hydride is added to a center that is electrophilic usually to unsaturated carbon.
  • Some hydrides are needed in organic synthesis which serves as strong bases. These include sodium hydride, potassium hydride, and so on. In this, the hydride will act as a weak Bronsted acid which releases H2.
  • Calcium hydride is one example of hydrides and is used as desiccants, which helps in the removal of trace water present in organic solvents. This hydride will react with hydrogen that forms water and also hydroxide salt.
  • Hydrides are also used in energy storage technologies e.g., nickel-metal hydride batteries, metal hydrides for storing hydrogen fuel which is required in hydrogen-powered vehicles as well as other purposes related to the hydrogen economy.
  • Hydride complexes can act as catalysts as well as catalytic intermediates needed in various homogeneous and heterogeneous catalytic activities. Some examples of such processes or activities include hydrosilylation, hydrogenation, hydroformylation, and hydrodesulfurization.
  • There are some enzymes that operate through the hydride intermediates (e.g. hydrogenase).

Types of Hydrides

There are various types of hydrides and they are usually divided into 3 groups. This is based on the bond between the element and the hydrogen or in simple terms the chemical bonding. These three types are as follows:

  1. Covalent Hydrides

When there is a reaction between hydrogen and other electronegative elements which are similar then covalent hydrides are formed. Examples of these electronegative elements are C, Si, and so on. Some examples of the hydrides formed are CH4 and NH3. The compounds formed during the reaction of hydrogen and non-metals are usually covalent hydrides. These compounds can be volatile or non-volatile in nature. Covalent hydrides can also be either liquids or gases apart. An example of a covalent Hydride is silane (SiH4).

Quiz: Which compound is covalent hydrides?

Answer Options:

1)CH4  2)CsH  3)HCL  4)NaH

  • Ionic hydrides Saline Hydrides

The ionic hydrides are also known as the saline hydrides exist when there is a reaction between a hydrogen molecule and highly electropositive elements belonging to the s-block. These usually consist of Alkali Metals and Alkaline Earth Metals. The hydrides are crystalline when they are in the solid-state and are non-volatile and non-conducting. In the liquid state, however, these ionic hydrides conduct electricity. During electrolysis, this type of hydride liberates hydrogen gas generally at the given anode.  It is difficult for them to dissolve in conventional solvents. Some common applications of this type of hydrides are as reducing reagents as well as bases in the synthesis of organic chemicals. Here are some examples of such Hydrides: CaH2, KH, etc. The hydrogen here exists as H (negatively charged ion).

  • Metallic hydrides

When hydrogen forms a bond with metals then this type of hydride is known as a metal hydride. The bond formed is normally of covalent type however, at times they are formed by ionic bonds. The most commonly involved ones are the transition metals. Most of these have high boiling and melting points, are non-stoichiometric, and are hard in nature E.g. TiH other metals can be aluminium, magnesium, cadmium, and so on.

More about covalent hydrides

These are usually formed with non-metals as discussed in the section above. Here one or more non-metals combine with hydrogen forming a compound. Here the electron pairs are shared by hydrogen with an electropositive product, thus forming a covalent bond. An example of this is when hydrogen forms a bond with chlorine which results in the formation of hydrochloric acid (HCl ).

H2(g)+Cl2(g) 2HCl (g)

3H2(g) +N2(g)→ 2NH3(g)

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