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. 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, 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 sodium borohydride, lithium aluminium hydride,
diisobutylaluminium hydride and super hydride, are commonly used as reducing
agents in chemical synthesis. The hydride adds to an electrophilic center,
typically unsaturated carbon. Hydrides such as sodium hydride and
potassium hydride are used as strong bases in organic synthesis. The hydride
reacts with the weak Bronsted acid releasing H2.
Hydrides such as calcium hydride are used as desiccants, i.e. drying agents,
to remove trace water from organic solvents. The hydride reacts with water
forming hydrogen and hydroxide salt. The dry solvent can then be distilled or vac
transferred from the "solvent pot". Hydrides are important in storage
battery technologies such as Nickel-metal hydride battery. Various
metal hydrides have been examined for use as a means of hydrogen storage for
fuel cell-powered electric cars and other purposed aspects of a hydrogen
economy. Hydride complexes are catalysts and
catalytic intermediates in a variety of homogeneous and heterogeneous catalytic
cycles. Important examples include hydrogenation, hydroformylation,
hydrosilylation, hydrodesulfurization catalysts. Even certain enzymes, the
hydrogenase, operate via hydride intermediates. The energy carrier NADH
reacts as a hydride donor or hydride equivalent.
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 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 forms one or more hydrides. These
substances have been classified into three main types according to the nature
of their bonding: 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 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.
C6H5C(O)CH3 + KH → C6H5C(O)CH2K + H2 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. NaH + H2O → H2 + NaOH ΔH = −83.6 kJ/mol,
ΔG = −109.0 kJ/mol Often alkali metal hydrides react with
metal halides. Lithium aluminium hydride arises from reactions of lithium hydride
with aluminium chloride. 4 LiH + AlCl3 → LiAlH4 + 3 LiCl
= Covalent hydrides = 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 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.
Molecular hydrides often involve additional ligands such as,
diisobutylaluminium hydride 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 and lithium
aluminium hydride and hindered reagents such as DIBAL.
= Interstitial hydrides or metallic hydrides =
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. 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 hydride complexes =
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. 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. = Deuterides =
Hydrides containing deuterium are known as deuterides. Some deuterides, such as
LiD, are important fusion fuels in thermonuclear weapons.
Appendix on nomenclature 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 following is a list of the nomenclature for the hydride derivatives
of main group compounds according to this definition:
alkali and alkaline earth metals: metal hydride
boron: borane, BH3 aluminium: alumane, AlH3
gallium: gallane, GaH3 indium: indigane, InH3
thallium: thallane, TlH3 carbon: alkanes, alkenes, alkynes, and
all hydrocarbons silicon: silane
germanium: germane tin: stannane
lead: plumbane nitrogen: ammonia, hydrazine
phosphorus: phosphine arsenic: arsine
antimony: stibine bismuth: bismuthine
helium: helium hydride According to the convention above, the
following are "hydrogen compounds" and not "hydrides":
oxygen: water, hydrogen peroxide sulfur: hydrogen sulfide synonym: sulfur
hydride selenium: hydrogen selenide
tellurium: hydrogen telluride halogens: hydrogen halides
Examples: nickel hydride: used in NiMH batteries
palladium hydride: electrodes in cold fusion experiments
lithium aluminium hydride: a powerful reducing agent used in organic chemistry
sodium borohydride: selective specialty reducing agent, hydrogen storage in fuel
cells sodium hydride: a powerful base used in
organic chemistry diborane: reducing agent, rocket fuel,
semiconductor dopant, catalyst, used in organic synthesis; also borane,
pentaborane and decaborane arsine: used for doping semiconductors
stibine: used in semiconductor industry phosphine: used for fumigation
silane: many industrial uses, e.g. manufacture of composite materials and
water repellents ammonia: coolant, fuel, fertilizer, many
other industrial uses hydrogen sulfide: component of natural
gas, important source of sulfur Chemically, even water and hydrocarbons
could be considered hydrides. 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.
= Precedence convention = According to IUPAC convention, by
precedence, hydrogen falls between group 15 and group 16 elements. Therefore we
have NH3, 'nitrogen hydride', versus H2O, 'hydrogen oxide'.
See also Parent hydride
Hydron Hydronium
Proton Hydrogen ion
Hydride compressor References
Bibliography W. M. Mueller, J. P. Blackledge, G. G.
Libowitz, Metal Hydrides, Academic Press, N.Y. and London,
External links