Hydrides – Definition, Types, Uses, Examples

Any group of chemical compounds in which hydrogen is joined with another element is called a hydride. On the basis of the type of chemical connection involved, three fundamental types of hydrides may be distinguished: saline (ionic), metallic, and covalent. On the basis of structure, the fourth form of hydride, dimeric (polymeric) hydride, may be recognised (see borane). Nonconductors such as aluminium, copper, and beryllium hydrides occur in solid, liquid, and gaseous forms. All of them are thermally unstable, and some of them will explode if they come into touch with air or moisture.

What are Hydrides?

Hydride is the formal name for the anion of hydrogen, H^–. The phrase is used in a broad sense. At one extreme, any compounds containing covalently bound H atoms are referred to be hydrides: water is an oxygen hydride, ammonia is a nitrogen hydride, and so on. 

Hydrides are molecules and ions that have hydrogen covalently bonded to a less electronegative element. In such circumstances, the H core possesses a nucleophilic property, as opposed to acids’ protic nature. The hydride anion is extremely rare.

Hydrogen’s bonds with the other elements range from extremely to slightly covalent. Some hydrides, such as boron hydrides, defy standard electron-counting principles and their bonding is explained in terms of multi-centred bonds, whereas interstitial hydrides frequently entail metallic bonding. Hydrides can take the form of single molecules, oligomers or polymers, ionic solids, chemisorbed monolayers, bulk metals (interstitial), or other materials. While most hydrides respond as Lewis bases or reducing agents, some metal hydrides operate as hydrogen-atom donors and hence as acids.

Types of Hydrides

Hydrides are classified into three categories or groupings. The categories are determined by the elements with which the hydrogen makes bonds, or simply by chemical bonding. Ionic, covalent, and metallic hydrides are the three types of hydrides.

Ionic or Saline Hydrides

When a hydrogen molecule combines with highly electropositive s-block components, they produce (Alkali Metals and Alkaline Earth Metals). 

Ionic hydrides are crystalline, non-conducting, and non-volatile in solid form. They do, however, conduct electricity when they are liquid. When ionic hydrides are electrolyzed, hydrogen gas is liberated at the anode. Because saline or ionic hydrides do not dissolve in common solvents, they are usually used as bases or reducing reagents in chemical synthesis. 

NIH is an example.

When pure, these compounds are white crystalline solids, however, they are frequently grey due to trace metal impurities. According to structural investigations, these compounds contain a hydride anion, H^–, with a crystallographic radius that varies depending on the metal identity but is intermediate to that of the fluoride ion, F^–. Because saline hydrides react vigorously with water, emitting considerable amounts of gaseous hydrogen, they are useful as light, portable hydrogen sources.

Beryllium and magnesium, both alkaline-earth metals, can also create stoichiometric MH₂ hydrides. These hydrides, on the other hand, are more covalent in character. Although pure BeH₂ is difficult to isolate, its structure is considered to be polymeric with bridging hydrogen atoms. Sodium hydride, NaH, and calcium hydride, CaH₂, are two further examples of binary saline hydrides. Lithium aluminium hydride, LiAlH₄, and sodium borohydride, NaBH₄, are both commercial compounds used as reducing agents and are examples of complex saline hydrides (substances that provide electrons in oxidation-reduction reactions).

Covalent Hydrides

Silane is created when hydrogen combines with other related electronegative elements such as Si, C, and so on. CH₄ and NH₃ are two of the most common examples. Covalent hydrides are compounds generated when hydrogen reacts with non-metals in general. The chemicals share a covalent link and can be volatile or non-volatile. Covalent hydrides are both liquids and gases.

The majority of nonmetal hydrides are volatile compounds with weak van der Waals intermolecular interactions that hold them together in condensed form. Unless their properties are adjusted by hydrogen bonding, covalent hydrides are liquids or gases with low melting and boiling points (as in water). Covalent hydrides can be made from the periodic elements boron (B), aluminium (Al), and gallium (Ga). Boron undergoes a complex succession of hydrides.

As the periodic table is moved from group 13 to group 17, the nonmetal hydrogen compounds become more acidic and less hydridic in nature. That is, as they age, they become less capable of providing H^– and more prone to contribute H⁺. Carbon has the most comprehensive class of hydrogen compounds of any element in the periodic table in group 14. All of the other elements in group-14 form hydrides that are neither good H⁺ donors nor good H⁺ donors.

Each halogen creates a binary compound, HX, with hydrogen. At room temperature and pressure, these compounds are gases, with hydrogen fluoride having the highest boiling point due to intermolecular hydrogen bonding. Hydrogen halides, like group 16, are proton donors in an aqueous solution. However, as a class, these molecules are much stronger acids. The acid strength of the HX compounds increases as one moves down the group, with HF being the weakest acid and HI being the most powerful proton donor. All hydrogen halides, with the exception of HF, dissolve in water to generate powerful acids. The difference in proton-donating ability between HF and the other HX compounds is attributable to a number of variables, one of which being the strong bond formed between hydrogen and fluorine.

Metallic Hydrides

A metal hydride is a hydrogen compound that reacts with another metal element to form a bond. The link is generally covalent, however, hydrides can also be produced using ionic bonds. These are typically generated by transition metals and are non-stoichiometric, hard, and have high melting and boiling points.

Metallic alloys like hydrides have some metal-like properties, such as lustre and high electrical conductivity. They have varying physical qualities, with some being more brittle and others being harder than the metals from which they are created. In nature, such substances are regarded as intermediates between salts and alloys. Metallic hydrides are made up primarily of protons (positive hydrogen ions) and metal atoms floating in an electron sea. The hydride’s shine and electrical conductivity are related to the relative freedom of electron mobility.

Metallic hydrides are created by combining hydrogen gas with metals or metal alloys. Compounds containing the most electropositive transition metals have received the most attention (the scandium, titanium, and vanadium families). Titanium, zirconium, and hafnium, for example, generate nonstoichiometric hydrides when they absorb hydrogen and release heat. These hydrides have the same chemical reactivity as the finely divided metal, remaining stable at room temperature but becoming reactive when heated in air or with acidic chemicals. They have the appearance of metal as well, being greyish black solids. The metal appears to be in a +3 oxidation state, with primarily ionic bonding. In some processes, such as metallurgy, these hydrides are used as reducing agents.

The structure can also be used to identify a fourth form of hydride, dimeric (polymeric) hydride. Aluminium and perhaps copper and beryllium hydrides are solid, liquid, or gaseous nonconductors. All of them are thermally unstable, and some of them explode when they come into touch with air or moisture.

Uses of Hydrides

• In chemical synthesis, hydrides such as sodium borohydride, DIBAL, and super hydride are often utilised as reducing agents. The hydride reacts with an electrophilic core, which is usually unsaturated carbon.
• In organic synthesis, hydrides such as sodium hydride and potassium hydride are utilised as strong bases. When the hydride combines with the weak Bronsted acid, it produces H₂.
• Desiccants, or drying agents, such as calcium hydride, are used to eliminate trace water from organic solvents. When the hydride combines with water, it produces hydrogen and hydroxide salt. The dry solvent can then be vacuum transferred or distilled from the solvent pot.
• Hydrides play a vital role in storage battery technologies like the nickel-metal hydride battery. Various metal hydrides have been investigated for use as hydrogen storage for fuel cell-powered electric vehicles and other components of a hydrogen economy.
• Hydride complexes act as catalysts and intermediates in a wide range of homogeneous and heterogeneous catalytic cycles. Catalysts for hydrogenation, hydroformylation, hydrosilylation, and hydrodesulfurization are all important examples.

Sample Questions

Question 1: What is a hydride?

Answer:

Hydrides are molecules and ions that have hydrogen covalently bonded to a less electronegative element.

Question 2: What is the fourth type of hydride?

Answer:

On the basis of structure, a fourth form of hydride, dimeric hydride, is identified. Aluminum and perhaps copper and beryllium hydrides are solid, liquid, or gaseous nonconductors. All of them are thermally unstable, and some of them explode when they come into touch with air or moisture.

Question 3: What is the nature of most non-metal hydrides?

Answer:

The majority of nonmetal hydrides are volatile compounds with weak van der Waals intermolecular interactions that hold them together in condensed form. Unless their characteristics are changed by hydrogen bonding, covalent hydrides are liquids or gases with low melting and boiling points.

Question 4: How are saline hydrides formed?

Answer:

When a hydrogen molecule reacts with highly electropositive s-block elements, they are formed.

Question 5: What are some properties of metallic hydrides?

Answer:

Metallic alloylike hydrides have some metal-like properties, such as lustre and high electrical conductivity. They do, however, have a wide range of physical properties, with some being more brittle than the metals from which they are made and others being harder. In nature, such compounds are considered intermediates between salts and alloys.