Carbohydride

Carbohydrides (or carbide hydrides) are solid compounds in one phase composed of a metal with carbon and hydrogen in the form of carbide and hydride ions. The term carbohydride can also refer to a hydrocarbon.[1]

Not to be confused with hydrocarbon or carbohydrate.

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.[2]

Nomenclature

A carbodeuteride (or carbo-deuteride) is a compound where the hydrogen is of the isotope deuterium.[3][4]

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-propadiene) is produced.[5]

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, even though carbon or hydrogen have been proposed as alloying light elements in the core.[6]

Applications

Carbohydrides are studied for their ability in hydrogen storage.[7] 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.[8]

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.[7] The process is called self-propagating high-temperature synthesis or SHS.[9] A hydrocarbide may be formed when the metal is milled in a hydrocarbon, for example in the manufacture of titanium carbide.[8]

Rare earth carbohydrides can be prepared by heating a metal hydride with graphite in a closed metal container, with a hydrogen atmosphere.[10]

List
Name formula form space group unit cell appearance density structure ref
Lithium dicalcium tricarbide hydride Ca2LiC3H tetragonal P4mbm a=6.8236 c=3.7518 Z=2 silver 2.36 has C34− [5]
Titanium carbo-deuteride TiC0.48D0.60 cubic Fm3m a=4.30963 [7]
Titanium carbo-deuteride TiC0.48D0.60 trigonal Fm31 a=3.08208 c=5.0405 [7]
Zirconium carbohydride ZrC0.3H [11]
Hafnium carbohydride Hf2CH2 a=3.427 c=5.476 [11][12]
thorium carbohydride ThCH2 is cubic under 380°,

and above is hexagonal.

 [13][14]
Th2CH2 hexagonal a=3.083 c=5.042 [12]
Th3CH4 monoclinic [12]
Niobium carbohydride NbC0.76H0.18 [15]
Barium indium allenylide hydride Ba12InC18H4 cubic Im3 a=11.1447 InBa12 icosahedrons [16]
Y5Si3C0.5H7.33 [17]
La2C3H1.5 [18]
La2CH4 a=5.642 [19]
La2CH2 monoclinic C2/m a = 7.206, b = 3.932, c = 6.739, β = 94.66 ° [19]
La15(FeC6)4H hexagonal P6 a=8.7764 c=10.7355 Z=1 V=720.42 silver [20]
Ytterbium carbide hydride Yb2CH2 hexgonal a=3.575 c=5.786 [10]
Ytterbium dicarbide hydride Yb2C2H cubic a=4.974 fcc [10]
Pr3Fe27.5Ti1.5CxH monoclinic A2/m [21]
Dy2Co17C0.2H2.8 P63/mmc a=8.418 c=8.165 V=501.1 [22]
Dy2Ni17C0.4H2.7 P63/mmc a=8.3789 c=8.054 V=489.7 [22]
Gd2ICH P63/mmc a = 3.8128 c = 14.844 grey 8.071 [23]
Gd2BrCH P63/mmc grey [23]
Gd2ClCH P63/mmc grey [23]
Tb2ICH P63/mmc grey [23]
Tb2BrCH P63/mmc grey [23]

References
  1. Ure, Andrew (1867). Ures̓ Dictionary of Arts, Manufactures and Mines: Containing a Clear Exposition of Their Principles and Practice. Longmans, Green and Company. pp. 648–649.
  2. Rundqvist, Stig; Tellgren, Roland; Andersson, Yvonne (August 1984). "Hydrogen and deuterium in transition metal-p element compounds: Crystal chemical aspects of interstitial solid solubility and hydride phase formation". Journal of the Less Common Metals. 101: 145–168. doi:10.1016/0022-5088(84)90092-4.
  3. Makovec, M.; Ban, Z. (June 1970). "The crystal structure of thorium carbohydrides". Journal of the Less Common Metals. 21 (2): 169–180. doi:10.1016/0022-5088(70)90115-3.
  4. Renaudin, G.; Yvon, K.; Dolukhanyan, S.K.; Aghajanyan, N.N.; Shekhtman, V.Sh. (August 2003). "Crystal structures and thermal properties of titanium carbo-deuterides as prepared by combustion synthesis". Journal of Alloys and Compounds. 356–357: 120–127. doi:10.1016/S0925-8388(03)00107-5.
  5. Lang, David A.; Zaikina, Julia V.; Lovingood, Derek D.; Gedris, Thomas E.; Latturner, Susan E. (2010-12-15). "Ca 2 LiC 3 H: A New Complex Carbide Hydride Phase Grown in Metal Flux". Journal of the American Chemical Society. 132 (49): 17523–17530. doi:10.1021/ja107436n. ISSN 0002-7863. PMID 21090715.
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  14. Makovec, M.; Ban, Z. (December 1970). "The crystal structure of thorium carbohydrides part II. Hexagonal thorium carbohydride". Journal of the Less Common Metals. 22 (4): 383–388. doi:10.1016/0022-5088(70)90125-6.
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  16. Blankenship, Trevor V.; Dickman, Matthew J.; van de Burgt, Lambertus J.; Latturner, Susan E. (2015-02-02). "Ca 12 InC 13– x and Ba 12 InC 18 H 4 : Alkaline-Earth Indium Allenylides Synthesized in AE/Li Flux (AE = Ca, Ba)". Inorganic Chemistry. 54 (3): 914–921. doi:10.1021/ic502315m. ISSN 0020-1669. PMID 25375309.
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