Group 14 hydride

Group 14 hydrides are chemical compounds composed of hydrogen atoms and group 14 atoms (the elements of group 14 are carbon, silicon, germanium, tin, lead and flerovium).

Methane, silane, and germane, three simple group 14 hydrides

Tetrahydrides

The tetrahydride series has the chemical formula XH₄, with X representing any of the carbon family. Methane is commonly the result of the decomposition of organic matter and is a greenhouse gas. The other hydrides are generally unstable, poisonous metal hydrides.

They take on a pyramidal structure, and as such are not polar molecules like the other p-block hydrides.

Unlike other light hydrides such as ammonia, water and hydrogen fluoride, methane does not exhibit any anomalous effects attributed to hydrogen bonding, and so its properties conform well to the prevailing trend of heavier group 14 hydrides.

Compound	Chemical formula	Molecular geometry	Space-filling model
carbon tetrahydride hydrogen carbide methane (carbane)	CH4
silicon tetrahydride hydrogen silicide (silane)	SiH4
germanium tetrahydride hydrogen germanide (germane)	GeH4
tin tetrahydride hydrogen stannide (stannane)	SnH4
lead tetrahydride hydrogen plumbide (plumbane)	PbH4
flerovium tetrahydride hydrogen flerovide (flerovane)	FlH4

Hexahydrides

This series has the chemical formula X₂H₆. Ethane is commonly found alongside methane in natural gas. The other hydrides of the chemical formula X₂H₆ are less stable than the corresponding tetrahydrides XH₄, and they are more and more less stable as X goes from carbon (ethane C₂H₆ is stable) down to lead (or flerovium) in the periodic table (diplumbane Pb₂H₆ is unknown).

Compound	Chemical formula	Molecular geometry	Space-filling model
Ethane (dicarbon hexahydride) (dicarbane)	C2H6
Disilane (disilicon hexahydride)	Si2H6
Digermane (digermanium hexahydride)	Ge2H6
Distannane (ditin hexahydride)	Sn2H6
Diplumbane (dilead hexahydride)	Pb2H6
Diflerovane (diflerovium hexahydride)	Fl2H6

Higher group 14 hydrides

All straight-chain saturated group 14 hydrides follow the formula X_nH_2n+2, the same formula for the alkanes.

Cubane, a Platonic hydrocarbon and prismane

Many other group 14 hydrides are known. Carbon forms a huge variety of hydrocarbons (among the simplest alkanes are methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀, pentane C₅H₁₂ and hexane C₆H₁₄, with a wide range of uses. There is also polyethylene (CH₂)_n, where n is very large, a stable hydrocarbon polymer, the most commonly produced plastic. Hydrocarbons also include alkenes, which contain a double bond between carbon atoms (e.g. ethylene H₂C=CH₂), alkynes, which contain a triple bond between carbon atoms (e.g. acetylene H−C≡C−H), cyclic and branched hydrocarbons (e.g. cyclohexane C₆H₁₂, limonene C₁₀H₁₆, which is a cyclic hydrocarbon with double bonds between carbon atoms, and neopentane C(CH₃)₄, which is a branched hydrocarbon), as well as aromatic hydrocarbons such as benzene C₆H₆ and toluene C₆H₅−CH₃), whose study forms the core of organic chemistry.

Alongside hydrogen, carbon can form compounds with the chemically similar halogens, forming haloalkanes. The simplest of this series, the halomethanes, contain compounds such as dichloromethane CH₂Cl₂, chloroform CHCl₃ and iodoform CHI₃. Other such important chemicals include vinyl chloride H₂C=CHCl, which is used in the production of PVC.

The other group 14 elements have a lower tendency to catenate. Hydrosilicons (binary silicon-hydrogen compounds), a silicon analogs of hydrocarbons, such as silanes Si_nH_2n+2 are known for n = 1–8, in which thermal stability decreasing as n increases (e.g. silane SiH₄ and disilane Si₂H₆), as are cyclosilanes (e.g. cyclopentasilane Si₅H₁₀ and cyclohexasilane Si₆H₁₂). They are very reactive, pyrophoric colourless gases or volatile liquids. Their volatility is intermediate between the alkanes and the germanes. Unsaturated silanes, the silenes and silynes, have been characterized spectroscopically. The first members of each respectively are disilene H₂Si=SiH₂ and disilyne H−Si≡Si−H, the silicon analogues of ethylene and acetylene respectively.

The first five hydrogermaniums Ge_nH_2n+2 are known and are fairly similar to the hydrosilicones, e.g. germane GeH₄ and digermane Ge₂H₆. They are germanium analogues of alkanes.

Stannane SnH₄, a strong reducing agent slowly decomposes at room temperature to tin and hydrogen gas, and is decomposed by concentrated aqueous acids or alkalis; distannane, Sn₂H₆ is still more unstable, and longer hydrostannums (hydrotins) are unknown. Stannane and distannane are tin analogues of methane and ethane respectively.

Plumbane PbH₄ is very poorly characterised and is only known in trace amounts: even at low temperatures, synthesis methods that yield the other MH₄ compounds fail to give PbH₄. No other hydroplumbums (hydroleads) are known. However, some substituted diplumbanes, with a general chemical formula R₃Pb−PbR₃ are more stable, where the R groups are organyl.

Compounds containing hydrogen and multiple group 14 elements are known, one of the most famous of these being tetraethyllead Pb(CH₂CH₃)₄ which contains carbon and lead. The other examples are methylsilane H₃C−SiH₃ which contains carbon and silicon, tris(trimethylsilyl)germanium hydride ((CH₃)₃Si)₃GeH which contain carbon, silicon and germanium, silylgermane or germylsilane H₃Si−GeH₃ which contains silicon and germanium, and hexaphenyldiplumbane (C₆H₅)₃Pb−Pb(C₆H₅)₃ which contains carbon and lead.

See also

• Methylene CH₂
• Methylidyne CH
• Titanium(IV) hydride TiH₄, a structural analog of the group 14 tetrahydrides
• Zirconium hydride ZrH₄, ZrH₂ and others
• Zirconium(II) hydride ZrH₂
• Uranium(IV) hydride UH₄

Bibliography

• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.