Covalent Hydrides

There has been a perpetual debate on hydrides and their meaning. Hydrides may look simpler on the outside, but they are quite complex on the inside. Hydrides are compounds that involve the bonding of hydrogen atoms with other elements. They are helpful in the world of chemistry as they find applications in every walk of life. They have extensive laboratory usage as well as real-life usage. There are three main types of hydrides – ionic or saline hydrides, covalent hydrides, and metallic hydrides. 

Definition of Hydrides

Hydrides are compounds containing hydrogen atoms. Hydrides are the negative ions or the anion of the hydrogen atom, which is why they can form compounds of their own. The hydrogen atom is highly reactive and forms bonds with almost all the elements of the periodic table. The hydrides are formed due to the presence of a nucleophilic centre. It is observed in hydrides that the hydrogen atom is always bonded to the most electropositive element.

Hydrides are mostly covalent. They may also have multi-centred bonds with metallic bonding. They can be of many varieties – polymers, ionic solids, bulk metals, etc. Usually, hydrides are donated electrons, and they act as Lewis bases. Sometimes, they also act as reducing agents.

Types of Hydrides

The classification of hydrides depends on the types of bonds that are present with the elements. The three main types of hydrides are:

• Ionic or saline hydrides
• Covalent hydrides
• Metallic hydrides

Covalent Hydrides

Covalent hydrides are the compounds of hydrogen which form bonds with non-metals. These non-metals are a part of the p-block elements. They are electronegative. Covalent hydrides are joined by weak Van der Waals forces. Covalent hydrides consist of the elements of group 13. All the elements of group 13, including boron, gallium, and aluminium, form covalent hydrides. Among these, boron forms the most number of hydrides. This is why the hydrides of boron and aluminium are extensively used as hydrides sources. 

As we move from group 13 to group 17 of the periodic table, the hydrides become more acidic. This means that they become less capable of donating the anion of hydrogen. They are more likely to donate the cation of the hydrogen. It is said that carbon, which belongs to group 14, forms the most varieties of the periodic table. The elements after oxygen that react with hydrogen form toxic gases that are volatile. In group 16, the dihydrides are called weak acids. As the bond strength decreases, the ability to donate decreases as well. However, as the bond strength decreases, the acidity increases. This is why the reactivity of non-metal hydrides increases with the increase in atomic number.

Types of Covalent Hydrides

The covalent hydrides are divided into three categories according to their chemical bonding and electron number:

1.Electron-deficient hydrides

The electron-deficient hydrides are elements that bond with elements of group 13. They are Lewis acids. These are prone to accepting electrons instead of donating them, for example, boron hydride.

2.Electron-precise hydrides

Any hydride that is formed with elements that belong to group 14 is an electron-precise hydride. They have a tetrahedral structure, for example, hydrides of carbon.

3.Electron-rich hydrides

Hydrides that are formed with the elements from group 15 to group 17 are known as electron-rich hydrides. These hydrides have excess electrons in their valence shell. The excess electrons are present in lone pairs, for example, hydrides of nitrogen.

Properties of Covalent Hydrides

Various properties make the covalent hydrides different from other hydrides. The physical properties of covalent hydrides are mentioned as follows:

• They are both liquids and gases.
• There are non-volatile weak Van der Waals intermolecular interactions between them.
• They have low melting and boiling points.
• They are formed by boron, aluminium, and gallium.
• As we move across the group, the compounds become more acidic. The only exception is hydrogen fluoride.
• Hydrogen fluorides have the highest boiling point.
• All the elements of group 16 donate protons in an aqueous solution.
• These hydrides are strong acids; except hydrogen fluoride (HF), all hydrides are soluble in water. They form very strong acids.
• Lithium gallium hydride is a commonly used reducing agent.
• When these hydrides are in their pure form, they behave as crystalline solids.
• They are white and crystalline in pure state

Conclusion 

Hydrides are compounds containing hydrogen atoms. Hydrides are the negative ions or the anion of the hydrogen atom, which is why they can form compounds of their own. The hydrogen atom is highly reactive and forms bonds with almost all the elements of the periodic table. This is why there are so many readily available hydrides. Hydrides are mainly of three types – ionic or saline hydrides, covalent hydrides, and metallic hydrides. Covalent hydrides are the compounds of hydrogen which form bonds with non-metals. These non-metals are a part of the p-block elements. They are electronegative. As we move from group 13 to group 17 of the periodic table, the hydrides become more acidic. This means that they become less capable of donating the anion of hydrogen.