Disilyne is a silicon hydride with the formula . Several isomers are possible, but none are sufficiently stable to be of practical value. Substituted disilynes contain a formal silicon–silicon

triple bond A triple bond in chemistry is a chemical bond between two atoms involving six bonding electrons instead of the usual two in a covalent single bond. Triple bonds are stronger than the equivalent single bonds or double bonds, with a bond order o ...

and as such are sometimes written R₂Si₂ (where R is a substituent group). They are the silicon analogues of

alkyne \ce \ce Acetylene \ce \ce \ce Propyne \ce \ce \ce \ce 1-Butyne In organic chemistry, an alkyne is an unsaturated hydrocarbon containing at least one carbon—carbon triple bond. The simplest acyclic alkynes with only one triple bond and n ...

s. The term ''silyne'' has two diverse meanings. Some chemists use it to refer to compounds containing a silicon–silicon triple bond,Egon Wiberg, Arnold Frederick Holleman (2001) ''Inorganic Chemistry'', Elsevier whereas others use the term to refer to compounds containing a silicon–carbon triple bond by analogy to silene, which often refers to compounds containing silicon–carbon double bonds. The term polysilyne can refer to the layer polymer (SiH)_n or substituted derivatives.

Substituted disilynes

The first example isolated and characterised by X-ray crystallography is an emerald green crystalline compound reported in 2004. This molecule has the formula R'_2R''Si-Si#Si-SiR''R'_2, where R' = HC(SiMe₃)₂ and R'' = HCMe₂. It was prepared by the reduction of the related tetrabrominated precursor by

potassium graphite Graphite intercalation compounds are complex materials having a formula where the ion or is inserted ( intercalated) between the oppositely charged carbon layers. Typically ''m'' is much less than 1. These materials are deeply colored solids t ...

(KC₈). It is air- and moisture-sensitive but is a stable solid up to 128 °C. The geometry of the central four-silicon chain is unlike that of analogous carbon structures. Whereas substituted

s are linear, having a 180°

bond angle Bond or bonds may refer to: Common meanings * Bond (finance), a type of debt security * Bail bond, a commercial third-party guarantor of surety bonds in the United States * Chemical bond, the attraction of atoms, ions or molecules to form chemical ...

at each end of the carbon–carbon triple bond, the silicon chain is bent to 137° at each end. The four silicon atoms in the chain are however perfectly coplanar, with the first and fourth silicon atoms ''

trans Trans- is a Latin prefix meaning "across", "beyond", or "on the other side of". Used alone, trans may refer to: Arts, entertainment, and media * Trans (festival), a former festival in Belfast, Northern Ireland, United Kingdom * ''Trans'' (film ...

'' to one another. The central triple

bond length In molecular geometry, bond length or bond distance is defined as the average distance between nuclei of two bonded atoms in a molecule. It is a transferable property of a bond between atoms of fixed types, relatively independent of the rest of ...

is 206 pm, which is around 4% shorter than the typical bond-length of Si–Si double bonds (214 pm)) and the Si–Si single bonds are 237 pm. The colour is attributed to a weak π–π^* transition. Calculations show a

bond order In chemistry, bond order, as introduced by Linus Pauling, is defined as the difference between the number of bonds and anti-bonds. The bond order itself is the number of electron pairs (covalent bonds) between two atoms. For example, in diat ...

of 2.6. An alternative calculation of the bond order by a different group describes the bonding as essentially due to only two electron pairs, with the third pair in a

non-bonding orbital A non-bonding orbital, also known as ''non-bonding molecular orbital'' (NBMO), is a molecular orbital whose occupation by electrons neither increases nor decreases the bond order between the involved atoms. Non-bonding orbitals are often designat ...

. Reaction of this compound with

phenylacetylene Phenylacetylene is an alkyne hydrocarbon containing a phenyl group. It exists as a colorless, viscous liquid. In research, it is sometimes used as an analog for acetylene; being a liquid, it is easier to handle than acetylene gas. Preparation I ...

produced a 1,2 disila

benzene Benzene is an organic chemical compound with the molecular formula C6H6. The benzene molecule is composed of six carbon atoms joined in a planar ring with one hydrogen atom attached to each. Because it contains only carbon and hydrogen atoms, ...

. Other workers have also reported another related compound which contains a hexasila-3-yne chain: :R₃Si(SiR₃)SiMeSi₂SiMe(SiR₃)SiR₃ :where Me = methyl and R = ''t''-butyl In this compound, the Si–Si triple bond length was calculated as 207 pm.

Heavier group 14 analogues

Triple bonded compounds of the heavier members of

group 14 The carbon group is a group (periodic table), periodic table group consisting of carbon (C), silicon (Si), germanium (Ge), tin (Sn), lead (Pb), and flerovium (Fl). It lies within the p-block. In modern International Union of Pure and Applied Chem ...

have also been prepared;

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

, and

tin Tin is a chemical element with the symbol Sn (from la, stannum) and atomic number 50. Tin is a silvery-coloured metal. Tin is soft enough to be cut with little force and a bar of tin can be bent by hand with little effort. When bent, t ...

and

germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbors s ...

(

digermyne Digermynes are a class of compounds that are regarded as the heavier digermanium analogues of alkynes. The parent member of this entire class is HGeGeH, which has only been characterized computationally, but has revealed key features of the whole ...

) The cores of the disilyne, digermyne, distannyne, and diplumbyne have similarly bent geometries. These findings are generally consistent with the absence of conventional triple bonds.

References

{{Hydrides by group Silicon hydrides

Substituted disilynes

Heavier group 14 analogues

See also

References