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What are Covalent Hydrides?

Covalent hydrides are mainly hydrogen and non metallic compounds in which the bonds are clearly electron pairs shared by atoms of similar electronegativities (covalent bonding). Most nonmetal hydrides, for example, are volatile compounds kept together in the condensed state by weak intermolecular van der Waals interactions. Except in cases where their properties are changed by hydrogen bonding (such as water), covalent hydrides are liquids or gases with low melting and boiling points. 

NH3, H2O, and HF are the covalent hydrides examples. In these covalent hydrides examples, molecules are kept together in the liquid state, generally existing mainly by hydrogen bonded form, despite their volatile nature.

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Boron (B), aluminium (Al), and gallium (Ga) of group 13 in the periodic table can be used to make covalent hydrides. Boron forms a large number of hydrides. While some molecular hydrides like AlH3 and Ga2H6 have been identified and characterised to some extent, the neutral hydrogen compounds of aluminium and gallium remain elusive species. Both boron molecular hydrides (BH4) and aluminium molecular hydrides AlH4 ionic hydrogen species are widely used as hydride sources.

The hydrogen compounds of nonmetals become more acidic and less hydridic in nature as the periodic table progresses from group 13 to group 17. That is, they become less capable of donating Hydrogen and more likely to donate Hydronium ion as time goes by. Carbon has the largest class of hydrogen compounds of any element in the periodic table in group 14. All other elements in group 14 type hydrides that are neither good Hydronium ion nor good Hydrogen donors. This is also valid for group 15 hydrides. All of the elements in group 16 form hydrides. H2S, H2Se, and H2Te—hydrogen compounds produced with elements that come after oxygen—are all volatile, poisonous gases with repulsive odours. They're made by mixing dilute acid with the appropriate metal sulphide, selenide, or telluride. In water, all of the hydrides in group 16 behave as weak acids, with the acidity increasing as the family progresses. The hydride's ability to donate a hydrogen ion is directly proportional to the element-hydrogen bond's declining bond power. That is, as the bond strength of the family decreases, the acidity rises. The general chemical reactivity of nonmetal hydrides increases with increasing atomic number of the nonmetal for the same reason.

Each halogen reacts with hydrogen to form HX, a binary compound. These compounds are gases at room temperature and pressure, with hydrogen fluoride having the highest boiling point due to intermolecular hydrogen bonding. Hydrogen halides, including those in group 16, are proton donors in aqueous solution. These compounds, on the other hand, are much stronger acids as a group. The acid strength of the HX compounds increases as they progress through the group, with HF being the weakest acid and HI being the most powerful proton donor. Both hydrogen halides dissolve in water to form strong acids, with the exception of HF. The disparity in proton-donating potential between HF and the other HX compounds is due to a number of reasons, one of which is the tight hydrogen-fluorine bond.

Hydrides of Metal

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Two well-known reducing agents are group-13 hydridic anions. The tetrahydridoborate (borohydride) anion, BH4, and its derivatives, the tetrahydridoaluminate anion, AlH4, are among the most widely used reducing agents in chemistry. The most widely used cations are Na+ for BH4 (to form NaBH4) and Li+ for AlH4 to form LiBH4 or LiAlH4.  In both organic and inorganic reduction reactions, both compounds have particular applications. As a reducing agent, lithium gallium hydride (LiGaH4) may be used. Both of these compounds are white crystalline solids when pure, and their thermal and chemical stabilities are such that the boron compounds have higher thermal and chemical stabilities than the aluminium compounds, which in turn have higher thermal and chemical stabilities than the gallium compounds.

Types of Hydride

Covalent Hydrides 

These are the first major categories, which are formed when a hydrogen atom and one or more nonmetals combine to form a compound. When hydrogen shares electron pairs with a more electropositive product, it forms a covalent bond. Volatile and non-volatile hydrides are also possible. The term "volatile" simply refers to the ability to vaporize at low temperatures. As hydrogen bonds with chlorine to form hydrochloric acid, this is an example of a covalent hydride ( HCl ). Here are some examples:

H2(g)+Cl2(g)→ 2HCl (g)

3H2(g) +N2(g)→ 2NH3(g)

If you progress from group 13 to group 17, the nonmetal hydrides on the periodic table become more electronegative. This indicates that they are less capable of donating an electron and prefer to retain them as their electron orbital fills up. They will donate an H+ instead of a H since they are more acidic.

Ionic Hydrides

Ionic hydrides are the second form of hydride (also known as saline hydrides or pseudohalides). These compounds form when hydrogen reacts with the most active metals, especially group one and two alkali and alkaline-earth metals. The hydrogen in this group takes the form of the hydride ion ( H ). They form bonds with metal atoms that are more electropositive. Ionic hydrides are insoluble in liquids and are normally binary compounds (i.e., only two elements in the compound). Some ionic hydrides examples are LiH (lithium hydride), NaH (sodium hydride), and KH (potassium hydride)

2A (s) +H2 → 2AH (s)

A may be any 1 group metal.

A (s) +H2(g) → AH2 (s)

A may be any 2 group metal.

Hydrogen gas is generated when ionic hydrides react vigorously with water.

Metallic hydrides, also known as interstitial hydrides, are the third form of hydride. Transition metals form hydrogen bonds. The ionic hydrides properties are given below:

These hydrides have the fascinating and special property of being nonstoichiometric, which means that the proportion of H atoms to metals is not set. The composition of nonstoichiometric compounds varies. The concept and foundation for this is that with metal and hydrogen bonding, there is a crystal lattice that H atoms can and can fill in between, but this is not a definite ordered filling. As a result, the proportion of H atoms to metals is not constant. Metallic hydrides do, however, contain more simple stoichiometric compounds.

Intermolecular Interactions

Since at least one hydrogen atom is present, you would assume that hydrides are all intact due to hydrogen bonding, but this is not the case. Just a few hydrides are connected by hydrogen bonds. Hydrogen bonds have energies of the order of 15-40 kJ/mol, which are somewhat strong, but they are still much weaker than covalent bonds, which have energies greater than 150 kJ/mol. As hydrogen bonds come into contact with neighbouring molecules, they can be weak. Fluorine, oxygen, and nitrogen, in particular, are more susceptible to hydrogen bonding.

Hydrogen is bound to a strongly electronegative atom in hydrides, which gives them more distinct properties. As a result, the boiling points of ammonia (NH3), water (H2O), and hydrogen fluoride (HF) break the rising boiling point pattern in the chart below comparing the boiling points of groups 14-17 hydrides.

According to popular belief, as molecular mass rises, so do boiling points. The three following hydrides have high boiling points as a result of their hydrogen bonds, as opposed to the initial expectation of having the lowest boiling points. Because of the high ionic character of the compounds, heavy dipole-dipole attractions occur in these hydrogen bonds.

Uses of Hydrides

  • In chemical synthesis, hydrides including sodium borohydride, lithium aluminium hydride, diisobutylaluminium hydride (DIBAL), and super hydride are widely used as reducing agents. An electrophilic base, usually unsaturated carbon, is added by the hydride.

  • In organic synthesis, strong bases such as sodium hydride and potassium hydride are used. H2 is generated when the hydride reacts with the weak Bronsted acid.

  • Desiccants, or drying agents, such as calcium hydride are used to extract trace water from organic solvents.

  • Hydrogen and hydroxide salt are formed when the hydride reacts with water. The dry solvent in the "solvent pot" can then be distilled or vacuum transferred.

  • In storage battery technologies like the nickel-metal hydride battery, hydrides are crucial. Various metal hydrides have been investigated for use as hydrogen storage for fuel cell-powered electric vehicles and other applications.

  • In a number of homogeneous and heterogeneous catalytic cycles, hydride complexes act as catalysts and catalytic intermediates. Hydrogenation, hydroformylation, hydrosilylation, and hydrodesulfurization catalysts are only a few examples. Also some enzymes, such as hydrogenase, use hydride intermediates to work. Nicotinamide adenine dinucleotide, an energy carrier, reacts as a hydride donor or hydride equivalent.

Role of Hydride Ion

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Only under extreme conditions do free hydride anions occur, and they are not used in homogeneous solutions. Many molecules, on the other hand, have hydridic hydrogen centres.

Apart from electrodes, the hydride ion is the most basic anion, containing only two electrons and a proton. Hydrogen has a low electron affinity of 72.77 kJ/mol and is a strong Lewis base that interacts exothermically with protons.

H- + H+ → H2  ΔH for this reaction is −1676 kJ/mol

Did You Know?

  • HF is least volatile and HCl is the most volatile hydrogen halide amongst hydrogen halides.

  • The interstitial halides are the compound in which hydrogen enters the metallic lattice.

FAQ (Frequently Asked Questions)

Question1: What are Hydrides?

Answer: A hydride is formally the hydrogen anion, H, in chemistry. 1st The word is used in a broad sense. At one extreme, all compounds containing covalently bound H atoms are referred to as hydrides: water is an oxygen hydride, ammonia is a nitrogen hydride, and so on. Hydrides are compounds and ions in which hydrogen is covalently bound to a less electronegative substance, according to inorganic chemists. The H centre has a nucleophilic character in these situations, which compares with the protic character of acids. The hydride anion is a very uncommon ion. With the exception of He, Ne, Ar, Kr, Pm, Os, Ir, Rn, Fr, and Ra, almost all elements form binary compounds with hydrogen. Exotic molecules like positronium hydride have been developed as well.

Question2: What is the Type of Bond Present in the Hydrides?

Answer: The covalent bonds between hydrogen and the other elements vary from strong to weak. Some hydrides, such as boron hydrides, do not follow traditional electron-counting laws, and their bonding is characterised using multi-centered bonds, while interstitial hydrides often use metallic bonding. Discrete compounds, oligomers or polymers, ionic solids, chemisorbed monolayers, bulk metals (interstitial), and other materials may all be hydrides. Although most metal hydrides react as Lewis bases or reducing agents, some act as hydrogen-atom donors and thus as acids.

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