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Hydride

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Hydride

In chemistry, a Hydride is the anion of hydrogen, H, or, more commonly, it is 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 hydrogen bonded to metals or metalloid may also be referred to as hydrides, even though in this case the hydrogen centres can have a protic character. Almost all of the elements form binary compounds with hydrogen, the exceptions being He, Ne, Ar, Kr, Pm, Os, Ir, Rn, Fr, and Ra.[1][2][3][4]

Contents

  • Bonds 1
  • Applications 2
  • Hydride ion 3
  • Types of hydrides 4
    • Ionic hydrides 4.1
    • Covalent hydrides 4.2
    • Interstitial hydrides or metallic hydrides 4.3
    • Transition metal hydride complexes 4.4
    • Deuterides 4.5
  • Appendix on nomenclature 5
    • Precedence convention 5.1
  • See also 6
  • References 7
  • Bibliography 8
  • External links 9

Bonds

Bonds between hydrogen and other elements range from highly to somewhat covalent. Some hydrides, e.g. boron hydrides, do not conform to classical electron-counting rules, and the bonding is described in terms of multi-centered bonds, whereas the interstitial hydrides often involve metallic bonding. Hydrides can be discrete molecules, oligomers or polymers, ionic solids, chemisorbed 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.

Applications

  • Hydrides such as organic synthesis. The hydride reacts with the weak Bronsted acid releasing H2.
  • Hydrides such as hydrogen and hydroxide salt. The dry solvent can then be distilled or vac transferred from the "solvent pot".

Hydride ion

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 as a powerful Lewis base.

H + H+ → H2; ΔH = −1676 kJ/mol

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

H2 + 2e 2H; Eo = −2.25 V

Types of hydrides

According to the general definition every element of the periodic table (except some noble gases) forms one or more hydrides. These substances have been classified into three main types according to the nature of their bonding:[1]

  • Ionic hydrides, which have significant ionic bonding character.
  • Covalent hydrides, which include the hydrocarbons and many other compounds which covalently bond to hydrogen atoms.
  • Interstitial hydrides, which may be described as having metallic bonding.

While these divisions have not been used universally, they are still useful to understand differences in hydrides.

Ionic hydrides

Ionic or saline hydride, is a hydrogen atom bound to an extremely electropositive metal, generally an

External links

W. M. Mueller, J. P. Blackledge, G. G. Libowitz, Metal Hydrides, Academic Press, N.Y. and London, (1968)

Bibliography

  1. ^ a b Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
  2. ^ Concise Inorganic Chemistry J.D. Lee
  3. ^ Main Group Chemistry, 2nd Edition A.G. Massey
  4. ^ a b Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005 (Red Book) Par. IR-6 - Full text (PDF)
  5. ^
  6. ^ Brown, H. C. “Organic Syntheses via Boranes” John Wiley & Sons, Inc. New York: 1975. ISBN 0-471-11280-1.
  7. ^ A. Dedieu (Editor) Transition Metal Hydrides 1991, Wiley-VCH, Weinheim. ISBN 0-471-18768-2

References

See also

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).

Precedence convention

All metalloid hydrides are highly flammable. All solid non-metallic hydrides except ice are highly flammable. But,when Hydrogen combines with halogens, it produces acids rather than hydrides and they are not flammable.

Examples:

According to the convention above, the following are "hydrogen compounds" and not "hydrides":

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]

Protide, deuteride, and tritide are used to describe ions or compounds, which contain enriched hydrogen-1, deuterium or tritium, respectively.

Appendix on nomenclature

Hydrides containing deuterium are known as deuterides. Some deuterides, such as LiD, are important fusion fuels in thermonuclear weapons.

Deuterides

Transition metal hydrides include compounds that can be classified as covalent hydrides. Some are even classified as interstitial 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 examples from the growing collection of known 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.

Transition metal hydride complexes

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.

Interstitial hydrides most commonly exist within metals or alloys. They are traditionally termed 'compounds', even though they do not strictly conform to the definition of a compound; more closely resembling common alloys such as steel. In such hydrides, hydrogen can exist as either atomic, or diatomic entities. Mechanical or thermal processing, such as bending, striking, or annealing may cause the hydrogen to precipitate out of solution, by degassing. 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. Hydrides of this type form according to either one of two main mechanisms. The first mechanism involves the adsorption of dihydrogen, succeeded by the cleaving of the H-H bond, the delocalisation of the hydrogen's electrons, and finally, the diffusion of the protons into the metal lattice. The other main mechanism involves the electrolytic reduction of ionised hydrogen on the surface of the metal lattice, also followed by the diffusion of the protons into the lattice. The second mechanism is responsible for the observed temporary volume expansion of certain electrodes used in electrolytic experiments.

Interstitial hydrides or metallic hydrides

Molecular hydrides often involve additional ligands such as, sodium borohydride (NaBH4) and lithium aluminium hydride and hindered reagents such as DIBAL.

According to some definitions, covalent hydrides cover all other compounds containing hydrogen. Some definitions limit hydrides to hydrogen centres that formally react as hydrides, i.e. are nucleophilic, and hydrogen atoms bound to metal centers.These hydrides are formed by all the true non-metals (except zero group elements) and the elments like Al, Ga, Sn, Pb, Bi, Po, etc., which are normally metallic in nature, i.e., this class includes the hydrides of p-block elements. 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 important segment of covalent hydrides are complex metal hydrides, powerful soluble hydrides commonly used in synthetic procedures.

Covalent hydrides

4 LiH + AlCl3 → LiAlH4 + 3 LiCl

Often alkali metal hydrides react with metal halides. Lithium aluminium hydride (often abbreviated as LAH) arises from reactions of lithium hydride with aluminium chloride.

NaH + H2O → H2 (g) + NaOH ΔH = −83.6 kJ/mol, ΔG = −109.0 kJ/mol

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.

C6H5C(O)CH3 + KH → C6H5C(O)CH2K + H2

[6]

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