In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group. Compounds containing metal or metalloid bonds to hydrogen are often referred to as hydrides, even though these hydrogen centres can have a protic character. Almost all of the elements form binary compounds with hydrogen, the exceptions being the noble gases and Mn, Fe, Co, Mo, Tc, Ru, Rh, Ag, W, Re, Os, Ir, Pt, Au, Fr, Ra, Pm, and some of the actinides.[1][2][3][4]
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Bonds between hydrogen and other elements range from highly covalent to somewhat ionic. Hydride compounds often do not conform to classical electron-counting rules, but are described as well as multi-centered bonds and metallic bonding. Hydrides can be components of discrete molecules, oligomers or polymers, ionic solids, chemisorped monolayers, bulk metals (interstitial), and other materials. While hydrides traditionally react as Lewis bases or reducing agents, some metal hydrides behave as hydrogen-atom donors and as acids.
Free hydride anions exist only under extreme conditions and are not invoked for homogeneous solution. Instead, many compounds have hydrogen centres with hydridic character.
Aside from electride, the hydride ion is the simplest possible anion, consisting of two electrons and a proton. Hydrogen has a relatively low electron affinity, 72.77 kJ/mol and reacts exothermically with protons a powerful Lewis base.
The low electron affinity of hydrogen and the strength of the H–H bond (∆HBE = 436 kJ/mol) means that the hydride ion would also be a strong reducing agent
According to the general definition every element of the periodic table (except some noble gases) forms one or more hydrides. These compounds have been classified into three main types according to the nature of their bonding:[1]
While these divisions have not been used universally, they are still useful to understand differences in hydrides.
Ionic or saline hydride, is a hydrogen atom bound to an extremely electropositive metal, generally an alkali metal or alkaline earth metal. In these materials the hydrogen atom is viewed as a pseudohalide. Saline hydrides are insoluble in conventional solvents, reflecting their nonmolecular structures. Most ionic hydrides exist as "binary" materials involving only two elements including hydrogen. Ionic hydrides are used as heterogeneous bases and reducing reagents in organic synthesis.[6]
Typical solvents for such reactions are ethers. Water and other protic solvents cannot serve as a medium for ionic hydrides because the hydride ion is a stronger base than hydroxide and most hydroxyl anions. Hydrogen gas is liberated in a typical acid-base reaction.
Often alkali metal hydrides react with metal halides. Lithium aluminium hydride (often abbreviated as LAH) arises from reactions of lithium hydride with aluminium chloride.
According to the antiquated definition of hydride covalent hydrides cover all other compounds containing hydrogen. The more contemporary definition limits hydrides to hydrogen atoms that formally react as hydrides and hydrogen atoms bound to metal centers. In these substances the hydride bond is formally a covalent bond much like the bond made by a proton in a weak acid. This category includes hydrides that exist as discrete molecules, polymers or oligomers, and hydrogen that has been chem-adsorbed to a surface. A particularly import segment of covalent hydrides are complex metal hydrides, powerful soluble hydrides commonly used in synthetic procedures.
Molecular hydrides often involve additional ligands such as, diisobutylaluminium hydride (DIBAL) consists of two aluminum centers bridged by hydride ligands. Hydrides that are soluble in common solvents are widely used in organic synthesis. Particularly common are sodium borohydride (NaBH4) and lithium aluminium hydride and hindered reagents such as DIBAL.
Interstitial hydrides most commonly exist within metals or alloys. Their bonding is generally considered metallic. Such bulk transition metals form interstitial binary hydrides when exposed to hydrogen. These systems are usually non-stoichiometric, with variable amounts of hydrogen atoms in the lattice. In materials engineering, the phenomenon of hydrogen embrittlement results from the formation of interstitial hydrides.
Palladium absorbs up to 900 times its own volume of hydrogen at room temperatures, forming palladium hydride. This material has been discussed as a means to carry hydrogen for vehicular fuel cells. Interstitial hydrides show certain promise as a way for safe hydrogen storage. During last 25 years many interstitial hydrides were developed that readily absorb and discharge hydrogen at room temperature and atmospheric pressure. They are usually based on intermetallic compounds and solid-solution alloys. However, their application is still limited, as they are capable of storing only about 2 weight percent of hydrogen, insufficient for automotive applications.
Transition metal hydrides include compounds that can be classified as covalent hydrides. Some are even classified as interstial hydrides and other bridging hydrides. Classical transition metal hydride feature a single bond between the hydrogen centre and the transition metal. Some transition metal hydrides are acidic, e.g., HCo(CO)4 and H2Fe(CO)4. The anions [ReH9]2− and [FeH6]4− are rare examples of a molecular homoleptic metal hydrides.[7] As pseudohalides, hydride ligands are capable of bonding with positively polarized hydrogen centres. This interaction, called dihydrogen bond is similar to hydrogen bonding which exists between positively polarized protons and electronegative atoms with open lone pairs.
Protide, deuteride, and tritide are used to describe ions or compounds, which contain enriched hydrogen-1, deuterium or tritium, respectively.
In the classic meaning, hydride refers to any compounds hydrogen forms with other elements, ranging over groups 1–16 (the binary compounds of hydrogen). The following is a list of the nomenclature for the hydride derivatives of main group compounds according to this definition:[4]
According to the convention above, the following are "hydrogen compounds" and not "hydrides":
Examples:
A notable thing is that all solid non-metallic & metalloid hydrides are highly flammable. But,when Hydrogen combines with halogens, it produces acids rather than hydrides and they are not flammable.
According to IUPAC convention, by precedence (stylized electronegativity), hydrogen falls between group 15 and group 16 elements. Therefore we have NH3, 'nitrogen hydride' (ammonia), versus H2O, 'hydrogen oxide' (water).