Disilyne is a low valent silicon compound with the chemical formula Si<sub>2</sub>R<sub>2</sub> where oxidation state of Si is +1. Several isomers are possible, but none are sufficiently stable to be of practical value. Substituted disilynes contain a formal siliconâÂÂsilicon triple bond and as such are sometimes written R<sub>2</sub>Si<sub>2</sub> (where R is a substituent group). They are the silicon analogues of alkynes.
The term silyne has two diverse meanings. Some chemists use it to refer to compounds containing a siliconâÂÂsilicon triple bond, by analogy to the carbonâÂÂcarbon triple bond in alkynes, whereas others use the term to refer to compounds containing a siliconâÂÂcarbon triple bond by analogy to silene, which often refers to compounds containing siliconâÂÂcarbon double bonds. The term polysilyne can refer to the layer polymer (SiH)<sub>n</sub> or substituted derivatives.
The first substituted disilyne to be isolated and characterised by X-ray crystallography is one with an additional trisubstituted silicon group on each silicon of the disilyne core. The structure , where Râ² = HC(SiMe<sub>3</sub>)<sub>2</sub> and Râ²â² = HCMe<sub>2</sub>, is an emerald green crystalline compound reported in 2004.
It was prepared by the reduction of the related tetrabrominated precursor by potassium graphite (KC<sub>8</sub>). It is air- and moisture-sensitive but is a stable solid up to 128 ðC.
The geometry of disilynes is unlike that of analogous carbon structures. Whereas substituted alkynes, such as 2-butyne, are linear, having a 180ð bond angle at each end of the carbonâÂÂcarbon triple bond, the chain is bent to 137ð at each end. The four silicon atoms in the chain are however perfectly coplanar, with the first and fourth silicon atoms trans to one another. The central triple bond length is 206 pm, which is around 4% shorter than the typical bond-length of SiâÂÂSi double bonds (214 pm)) and the SiâÂÂSi single bonds are 237 pm. The color is attributed to a weak ÃÂâÂÂÃÂ<sup>*</sup> transition.
Calculations show a bond order 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. Reaction of this compound with phenylacetylene produced a 1,2-disilabenzene. Other workers have also reported another related compound which contains a hexasila-3-yne chain:
In this compound, the SiâÂÂSi triple bond length was calculated as 207 pm.
Triple bonded compounds of the heavier members of group 14 have also been prepared; lead, and tin and germanium (digermyne) 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.