Everything about Hydride totally explained
Hydride is the name given to the
negative ion of
hydrogen, H
−. Although this ion doesn't exist except in extraordinary conditions, the term hydride is widely applied to describe
compounds of hydrogen with other
elements, particularly those of
groups 1–16. The variety of compounds formed by hydrogen is vast, arguably greater than that of any other element. Various metal hydrides are currently being studied for use as a means of hydrogen storage in
fuel cell-powered electric cars and
batteries. They also have important uses in
organic chemistry as powerful
reducing agents, and many promising uses in
hydrogen economy.
Every element of the
periodic table (except some
noble gases) forms one or more hydrides. These compounds may be classified into three main types by the predominant nature of their
bonding:
- Saline hydrides, which have significant ionic character,
- Covalent hydrides, which include the hydrocarbons and many other compounds, and
- Interstitial hydrides, which may be described as having metallic bonding.
Hydride ion
» See also: hydrogen anion.
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, thus hydride is so basic that it's unknown in solution. This however is deceptive since the proton is so acidic it's also unknown in solution. The reactivity of the hypothetic hydride ion is dominated by its exothermic protonation to give
dihydrogen:
» :H
− + H
+ → H
2;
ΔH = −1676 kJ/mol
As a result, the hydride ion is one of the strongest
bases known. It would extract protons from almost any hydrogen-containing species. The low electron affinity of hydrogen and the strength of the H–H bond (436 kJ/mol) means that the hydride ion would also be a strong
reducing agent:
» :H
2 + 2e
− ⇌ 2H
−;
Eo = −2.25 V
Ionic hydrides
In ionic, or saline, hydrides, the hydrogen is viewed as a
pseudohalide. The saline hydrides are insoluble in conventional solvents, reflecting their nonmolecular structures. H
− has stable
electron configuration of
helium with a filled 1s-orbital. Ionic hydrides also feature an electropositive metal, usually one of the
alkali metals or
alkaline earth metals. These hydrides are called binary if they only involve two elements including hydrogen.
Chemical formulae for binary ionic hydrides typically MH (as in
LiH). As the charge on the metal increases, the M-H bonding becomes more covalent as in
MgH
2 and
AlH
3. Ionic hydrides are commonly encountered as basic
reagents in
organic synthesis:
» C6H5C(O)CH3 +
KH → C
6H
5C(O)CH
2K + H
2
Such reactions are heterogeneous because the KH doesn't dissolve. Typical solvents for such reactions are
ethers.
Water can't serve as a medium for pure ionic hydrides or LAH because the hydride ion is a stronger
base than
hydroxide. Hydrogen gas is liberated in a typical acid-base reaction.
» NaH + H
2O → H
2 (gas) + NaOH Δ
H = −83.6 kJ/mol,
ΔG = −109.0 kJ/mol
Alkali metal hydrides react with metal halides.
Lithium aluminium hydride (often abbreviated as LAH) arises from reactions with
aluminium chloride.
» 4
LiH + AlCl
3 → LiAlH
4 + 3 LiCl
Covalent hydrides
In covalent hydrides, hydrogen is
covalently bonded to more
electropositive element such as p-block (
boron,
aluminium, and Group 4-7) elements as well as
beryllium. Common compounds include the
hydrocarbons and
ammonia could be considered as
hydrides of
carbon and
nitrogen, respectively. Charge neutral covalent hydrides that are molecular are often volatile at room temperature and
atmospheric pressure. Some covalent hydrides are not volatile because they're polymeric—for example nonmolecular—such as the binary hydrides of aluminium and beryllium. Replacing some hydrogen atoms in such compounds with larger
ligands, one obtains molecular derivatives. For example,
diisobutylaluminium hydride (DIBAL) consists of two aluminium centers bridged by hydride ligands. Hydrides that are soluble in common solvents are widely used in
organic synthesis. Particularly common are
sodium borohydride (NaBH
4) and lithium aluminum hydride and hindered reagents such as DIBAL.
Transition metal hydrido complexes
Most transition
metal complexes form molecular compounds that contain one or more hydride ligands. Usually such compounds are discussed in the context of
organometallic chemistry. They are intermediates in many industrial processes that rely on metal catalysts, such as
hydroformylation,
hydrogenation, and
hydrodesulfurization.
Deprotonation of
dihydrogen complexes gives metal hydrides.
Two famous examples of transition metal hydrides are HCo(CO)
4 and H
2Fe(CO)
4, are acidic thus demonstrating that the term hydride is used very broadly. The anion is a rare example of a molecular
homoleptic metal hydride.
Interstitial hydrides of the transitional metals
Structurally related to the saline hydrides, the transition metals form binary hydrides which are often non-
stoichiometric, with variable amounts of hydrogen atoms in the lattice, where they can migrate through it. In
materials engineering, the phenomenon of
hydrogen embrittlement is a consequence of interstitial hydrides.
Palladium absorbs up to 900 times its own volume of hydrogen at room temperatures, forming
palladium hydride, and was therefore once thought as a means to carry hydrogen for vehicular
fuel cells. Hydrogen gas is liberated proportional to the applied temperature and pressure but not to the chemical composition.
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're capable of storing only about 2 weight percent of hydrogen, which isn't enough for automotive applications.
Nomenclature
The following is a list of the nomenclature for the hydride derivatives of main group compounds:
alkali and alkaline earth metals: metal hydride
boron: borane and rest of the group as metal hydride
carbon: alkanes, alkenes, alkynes, and all hydrocarbons
silicon: silane
germanium: germane
tin: stannane
lead: plumbane
nitrogen: ammonia ('azane' when substituted), hydrazine
phosphorus: phosphine ('phosphane' when substituted)
arsenic: arsine ('arsane' when substituted)
antimony: stibine ('stibane' when substituted)
bismuth: bismuthine ('bismuthane' when substituted)
According to the convention above, the following are "hydrogen compounds" and not "hydrides":
oxygen: water ('oxidane' when substituted), hydrogen peroxide
sulfur: hydrogen sulfide ('sulfane' when substituted)
selenium: hydrogen selenide ('selane' when substituted)
tellurium: hydrogen telluride ('tellane' when substituted)
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, for example manufacture of composite materials and water repellents
ammonia: coolant, fertilizer, many other industrial uses
hydrogen sulfide: component of natural gas, important source of sulfur
Chemically, even water and hydrocarbons could be considered hydrides.
Isotopes of hydride
Protide, deuteride, and tritide are used to describe ions or compounds, which contain enriched hydrogen-1, deuterium or tritium, respectively.
Precedence convention
According to IUPAC convention, by precedence (stylized electronegativity), hydrogen falls between group 15 and group 16 elements. Therefore we've NH3, 'nitrogen hydride' (ammonia), versus H2O, 'hydrogen oxide' (water).
Further Information
Get more info on 'Hydride'.
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