Carbohydrides (or carbide hydrides) are solid compounds in one phase composed of a metal with carbon and hydrogen in the form of

carbide In chemistry, a carbide usually describes a compound composed of carbon and a metal. In metallurgy, carbiding or carburizing is the process for producing carbide coatings on a metal piece. Interstitial / Metallic carbides The carbides of th ...

and

hydride In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride ...

ions. The term carbohydride can also refer to a

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

Structure and bonding

Many of the transition metal carbohydrides are non-stochiometric, particularly with respect to the hydrogen that can vary in proportion up to a theoretical balanced proportion. The hydrogen and carbon occupy holes in the metal crystalline lattice. The carbon takes up octahedral sites (surrounded by six metal atoms) and the hydrogen takes up tetrahedral sites in the metal lattice. The hydrogen atoms go to sites away from the carbon atoms, and away from each other, at least 2 Å apart, so there are no covalent bonds between the carbon or hydrogen atoms. Overall the lattice retains a high symmetry of the original metal.

Nomenclature

A carbodeuteride (or carbo-deuteride) is a compound where the hydrogen is of the isotope

deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...

Properties

Reactions

Metal carbide hydrides give off hydrogen when heated, and are in equilibrium with a partial pressure of hydrogen that depends on the temperature. When Ca₂LiC₃H is heated with ammonium chloride, the gas C₃H₄ (

methylacetylene Propyne (methylacetylene) is an alkyne with the chemical formula . It is a component of MAPD gas—along with its isomer propadiene (allene), which was commonly used in gas welding. Unlike acetylene, propyne can be safely condensed.Peter P� ...

propadiene Propadiene () or allene () is the organic compound with the formula . It is the simplest allene, i.e. a compound with two adjacent carbon double bonds. As a constituent of MAPP gas, it has been used as a fuel for specialized welding. Production ...

) is produced.

Comparisons

There are also metal cluster molecules and ions that contain both carbon and hydrogen. Methylidyne complexes contain the CH group with three bonds to a metal e.g. NiCH⁺ or PtCH⁺.

Natural occurrence

Iron carbide hydrides do not appear to be stable at the conditions present in the Earth's

inner core Earth's inner core is the innermost geologic layer of planet Earth. It is primarily a solid ball with a radius of about , which is about 20% of Earth's radius or 70% of the Moon's radius. There are no samples of Earth's core accessible for d ...

, even though carbon or hydrogen have been proposed as alloying light elements in the core.

Applications

Carbohydrides are studied for their ability in

hydrogen storage Hydrogen storage can be accomplished by several existing methods of holding hydrogen for later use. These include mechanical approaches such as using high pressures and low temperatures, or employing chemical compounds that release H2 upon demand ...

. Carbohydrides may be made when carbides are manufactured by milling, using hydrocarbons as a carbon source. Since the carbohydride is not the desired outcome, other material like graphite is added to try to maximise carbide production.

Preparation

Transition metal carbohydrides can be produced by heating a metal carbide in hydrogen, for example at 2000 °C and 3 bars. This reaction is exothermic, and just needs to be ignited at a much lower temperature. The process is called self-propagating high-temperature synthesis or SHS. A hydrocarbide may be formed when the metal is milled in a hydrocarbon, for example in the manufacture of titanium carbide. Rare earth carbohydrides can be prepared by heating a metal hydride with graphite in a closed metal container, with a hydrogen atmosphere.

List

References

{{Reflist Carbides Hydrides Mixed anion compounds