Hydrides: How They Form, Properties, Metallic and Non-Metallic Hydrides, Nomenclature and Examples

A hydride is hydrogen in its anionic form (H ^- ) or compounds that are formed from the combination of a chemical element (metallic or non-metallic) with the hydrogen anion. Of the chemical elements known, hydrogen is the simplest structure, because when it is in the atomic state it has a proton in its nucleus and an electron.

Despite this, hydrogen is only found in its atomic form under conditions of fairly high temperatures. Another way of recognizing hydrides is when it is observed that one or more central hydrogen atoms in a molecule have a nucleophilic behavior, as a reducing agent or even as a base.

Aluminum lithium hydride

Thus, hydrogen has the ability to combine with most elements of the periodic table to form different substances.

How are hydrides formed?

Hydrides are formed when hydrogen in its molecular form is associated with another element -either of metallic or non-metallic origin- directly by dissociating the molecule to form a new compound.

In this way, hydrogen forms bonds of covalent or ionic type, depending on the type of element with which it is combined. In the case of associating with transition metals, interstitial hydrides are formed with physical and chemical properties that can vary enormously from one metal to another.

The existence of free form hydride anions is limited to the application of extreme conditions that do not occur easily, so in some molecules the octet rule is not met.

It is possible that other rules related to the distribution of electrons are not given, having to apply expressions of links of multiple centers to explain the formation of these compounds.

Physical and chemical properties of hydrides

In terms of physical and chemical properties, it can be said that the characteristics of each hydride depend on the type of bond that is carried out.

For example, when the hydride anion is associated with an electrophilic center (generally it is an unsaturated carbon atom), the compound formed behaves as a reducing agent, whose use is very widespread in chemical synthesis.

In contrast, when combined with elements such as alkali metals, these molecules react with the weak acid (Bronsted acid) and behave as strong bases, releasing hydrogen gas. These hydrides are very useful in organic synthesis.

It is then observed that the nature of the hydrides is very varied, being able to form discrete molecules, solid of ionic type, polymers and many other substances.

For this reason they can be used as desiccants, solvents, catalysts or intermediates in catalytic reactions. They also have multiple uses in laboratories or industries for various purposes.

Metal hydrides

Metallic hydrides are those binary substances that are formed by the combination of a metallic element with hydrogen, generally one that is electropositive, such as alkaline or alkaline earth metals, although interstitial hydrides are also included.

This is the only type of reaction in which hydrogen (whose oxidation number is usually +1) has an extra electron at its outermost level; that is, its valence number is transformed to -1, although the nature of the bonds in these hydrides has not been completely defined by the discrepancy of the scholars of the subject.

Metal hydrides have some properties of metals, such as their hardness, conductivity and brightness; but unlike metals, hydrides have a certain fragility and their stoichiometry does not always comply with the weight laws of chemistry.

Non-metallic hydrides

This type of hydride arises from the covalent association between a non-metallic element and hydrogen, so that the non-metallic element is always in its lowest oxidation number to generate a single hydride with each one.

It also has that this type of compounds are, for the most part, gaseous in standard environmental conditions (25 ° C and 1 atm). For this reason, many non-metallic hydrides have low boiling points, due to the van der Waals forces, which are considered weak.

Some hydrides of this class are discrete molecules, others belong to the group of polymers or oligomers and even hydrogen that has gone through a chemisorption process on a surface can be included in this list.

How are hydrides named?

To write the formula for metal hydrides, start by writing the metal (the symbol of the metallic element) followed by hydrogen (MH, where M is the metal).

To name them starts with the word hydride followed by the name of the metal ("M hydride"), so LiH reads"lithium hydride", CaH ₂ it reads"calcium hydride"and so on.

In the case of non-metallic hydrides, the opposite is written for metallic hydrides; that is, it starts by writing the hydrogen (its symbol) happened by the non-metal (HX, where X is the non-metal).

To name them, start with the name of the non-metallic element and add the suffix"uro", ending with the words"hydrogen"("X-hydrogen uro"), so HBr reads"hydrogen bromide", H ₂ S reads"hydrogen sulfide"and so on.

Examples

There are many examples of metal and non-metal hydrides with different characteristics. Here are some mentioned:

Metal hydrides

- LiH (lithium hydride).

- NaH (sodium hydride).

- KH (potassium hydride).

- CsH (cesium hydride).

- RbH (rubidium hydride).

- BeH ₂ (Beryllium hydride).

- MgH ₂ (magnesium hydride).

- CaH ₂ (calcium hydride).

- SrH ₂ (strontium hydride).

- BaH ₂ (barium hydride).

Non-metallic hydrides

- HBr (hydrogen bromide).

- HF (hydrogen fluoride).

- HI (hydrogen iodide).

- HCl (hydrogen chloride).

- H ₂ S (hydrogen sulfide).

- H ₂ Te (hydrogen telluride).

- H ₂ Se (hydrogen selenide).

References

1. Wikipedia. (2017). Wikipedia. Retrieved from en.wikipedia.org
2. Chang, R. (2007). Chemistry (9th ed). McGraw-Hill.
3. Babakidis, G. (2013). Metal Hydrides. Recovered from books.google.co.ve
4. Hampton, M.D., Schur, D.V., Zaginaichenko, S.Y. (2002). Hydrogen Materials Science and Chemistry of Metal Hydrides. Recovered from books.google.co.ve

5. Sharma, R. K. (2007). Chemistry of Hidrydes and Carbides. Recovered from books.google.co.ve