View source for Chemistry:Silenes

{{short description|1=Silicon compounds with an Si=Si bond}}

[[File:Disilene-2D.svg|thumbnail|Structure of disilene, the simplest silene]]

In [[Chemistry:Inorganic chemistry|inorganic chemistry]], '''silenes''', or '''disilalkenes''',<ref name=Power>Philip P. Power "pi-Bonding and the Lone Pair Effect in Multiple Bonds between Heavier Main Group Elements" Chemical Reviews, 1999, 99, 3462. {{doi|10.1021/cr9408989}}</ref> are [[Chemistry:Silicon|silicon]] compounds that contain {{chem2|Si\dSi}} [[Chemistry:Double bond|double bond]]s. The [[Chemistry:Parent structure|parent molecule]] is [[Chemistry:Disilene|disilene]], {{chem2|Si2H4}}.

== Structure ==
The first transient disilene was reported in 1972 by D. N. Roark and Garry J. D. Peddle. Simple disilenes easily [[Chemistry:Polymerization|polymerize]]. To suppress this tendency, bulky [[Chemistry:Substituent|substituent]]s are used. Indeed the first isolable disilene, tetramesityldisilene, was described in 1981 by West, Fink, and Michl.<ref>West, R.; Fink, M. J.; Michl, J. ''Tetramesityldisilene, a Stable Compound Containing a Silicon-Silicon Double Bond'' Science 1981, Vol. 214, pp. 1343-1344. {{doi|10.1126/science.214.4527.1343}}.</ref><ref>{{Greenwood&Earnshaw2nd|p=363}}</ref> It was prepared by UV-photolysis of the related cyclic [[Chemistry:Trisilane|trisilane]]:
: 2 [Si(mesityl)<sub>2</sub>]<sub>3</sub> → 3 (mesityl)<sub>2</sub>Si=Si(mesityl)<sub>2</sub>

==Structure of tetramesityldisilene ==
Tetramesityldisilene {{chem2|(CH3)2Si\dSi(CH3)2}} is a yellow-orange solid. The Si=Si [[Chemistry:Covalent bond|double bond]] lengths of disilenes vary between 2.14 and 2.29 Å and are nearly 5 to 10% shorter than the Si-Si [[Chemistry:Covalent bond|single bond]] lengths of corresponding disilanes. A peculiarity of disilenes is the ''trans''-bending of the [[Chemistry:Substituent|substituent]]s, which is never observed in [[Chemistry:Alkene|alkene]]s. The ''trans''-bent angles of disilenes between the R<sub>2</sub>Si planes and the Si=Si vector range from 0 to 33.8 °. This distortion is rationalized by the stability of the corresponding [[Chemistry:Silylene|silylene]] fragments, although disilenes do not typically dissociate.

The distorted geometry of disilenes can be rationalized by considering the [[Chemistry:Valence|valence]] [[Physics:Electron configuration|orbital]]s of [[Chemistry:Silicon|silicon]], which are [[Physics:Electron configuration|3s]] and 3p, whereas those of [[Chemistry:Carbon|carbon]] are 2s and 2p. Thus, the energy gap between the [[Physics:Electron configuration|''n''s]] and [[Physics:Electron configuration|''n''p orbital]]s of a [[Chemistry:Silicon|silicon]] [[Physics:Atom|atom]] is larger than that of a [[Chemistry:Carbon|carbon]] [[Physics:Atom|atom]].
Therefore, [[Chemistry:Silylene|silylene]] fragments are in a [[Chemistry:Diradical|singlet]] state, while [[Chemistry:Carbene|carbene]] fragments are in a triplet state. So, when double bonds are formed by the interaction of these two fragments, disilenes which consist of two [[Chemistry:Silylene|silylene]] units are ''trans''-bending and [[Chemistry:Alkene|alkene]]s which consist of two [[Chemistry:Carbene|carbene]] units are planar.  The bending is even more extreme for the tin analogues of disilenes.<ref name=Power/>

==Synthesis==
Disilenes are generally synthesized by [[Chemistry:Redox|reduction]] of 1,2-dihalodisilane, by retro-Diels–Alder fragmentation, by [[Chemistry:Dimer|dimerization]] of [[Chemistry:Silylene|silylene]]s, by photofragmentation of cyclopolysilanes, or by rearrangement of silylsilylenes.

A series of 1,1,1,4,4,4-hexaalkyl-2,3-bis(trialkylsilyl)tetrasil-2-enes structural analogues are typically synthesised using reductive coupling of the corresponding 1,1,1,3,3,3-hexaalkyl-2,3-tribromotrisilanes.

== IUPAC names ==
To form the root of the IUPAC names for silenes, simply change the -an- infix of the parent to -en-. For example, '''{{Chem|SiH|3}}-{{Chem|SiH|3}}''' is the silane ''disilANe''. The name of '''{{Chem|SiH|2}}={{Chem|SiH|2}}''' is therefore ''disilENe''.

In higher silenes, where isomers exist that differ in location of the double bond, the following numbering system is used:
#Number the longest silicon chain that contains the double bond in the direction that gives the silicon atoms of the double bond the lowest possible numbers.
#Indicate the location of the double bond by the location of its first silicon.
#Name branched or substituted silenes in a manner similar to [[Chemistry:Silanes|silanes]].
#Number the silicon atoms, locate and name substituent groups, locate the double bond, and name the main chain.

==See also==
*[[Chemistry:Disilane|Disilane]]
*[[Chemistry:Disilyne|Disilyne]]
*[[Chemistry:Organosilicon|Organosilicon chemistry]]
*[[Chemistry:Double bond rule|Double bond rule]]
*[[Chemistry:Trisilaallene|Trisilaallene]]

==References==
{{reflist}}

[[Category:Silicon compounds]]

{{Sourceattribution|Silenes}}