inorganic chemistry Inorganic chemistry deals with chemical synthesis, synthesis and behavior of inorganic compound, inorganic and organometallic chemistry, organometallic compounds. This field covers chemical compounds that are not carbon-based, which are the subj ...

, silenes, or disilalkenes,Philip P. Power "pi-Bonding and the Lone Pair Effect in Multiple Bonds between Heavier Main Group Elements" Chemical Reviews, 1999, 99, 3462. are

silicon Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic lustre, and is a tetravalent metalloid (sometimes considered a non-metal) and semiconductor. It is a membe ...

compounds that contain

double bond In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betw ...

s, where the oxidation state of Si is +2. The parent molecule is disilene, Si₂H₄.

Structure

The first transient disilene was reported in 1972 by D. N. Roark and Garry J. D. Peddle. Simple disilenes easily

polymerize In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many form ...

. To suppress this tendency, bulky

substituent In organic chemistry, a substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. The suffix ''-yl'' is used when naming organic compounds that contain a single bond r ...

s are used. Indeed the first isolable disilene, tetramesityldisilene, was described in 1981 by

West West is one of the four cardinal directions or points of the compass. It is the opposite direction from east and is the direction in which the Sun sets on the Earth. Etymology The word "west" is a Germanic word passed into some Romance langu ...

, Fink, and Michl. It was prepared by UV-

photolysis Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by absorption of light or photons. It is defined as the interaction of one or more photons wi ...

of the related cyclic

trisilane Trisilane is the silane with the formula H2Si(SiH3)2. A liquid at standard temperature and pressure, it is a silicon analogue of propane. In contrast with propane, however, trisilane ignites spontaneously in air. __TOC__ Synthesis Trisilane was c ...

: : 2 i(mesityl)₂sub>3 → 3 (mesityl)₂Si=Si(mesityl)₂

Structure of tetramesityldisilene

Tetramesityldisilene is a yellow-orange solid. The Si=Si

lengths of disilenes vary between 2.14 and 2.29 Å and are nearly 5 to 10% shorter than the Si-Si

single bond In chemistry, a single bond is a chemical bond between two atoms involving two valence electrons. That is, the atoms share one pair of electrons where the bond forms. Therefore, a single bond is a type of covalent bond. When shared, each of th ...

lengths of corresponding disilanes. A peculiarity of disilenes is the ''trans''-bending of the

s, which is never observed in

alkene In organic chemistry, an alkene, or olefin, is a hydrocarbon containing a carbon–carbon double bond. The double bond may be internal or at the terminal position. Terminal alkenes are also known as Alpha-olefin, α-olefins. The Internationa ...

s. The ''trans''-bent angles of disilenes between the R₂Si planes and the Si=Si vector range from 0 to 33.8 °. This distortion is rationalized by the stability of the corresponding

silylene Silylene is a chemical compound with the formula SiR2. It is the silicon analog of carbene. Silylene rapidly when condensed. Silylenes are formal derivatives of silylene with its hydrogens replaced by other substituents. Most examples feature ...

fragments, although disilenes do not typically dissociate. The distorted geometry of disilenes can be rationalized by considering the valence orbitals of

, which are 3s and 3p, whereas those of

carbon Carbon () is a chemical element; it has chemical symbol, symbol C and atomic number 6. It is nonmetallic and tetravalence, tetravalent—meaning that its atoms are able to form up to four covalent bonds due to its valence shell exhibiting 4 ...

are 2s and 2p. Thus, the energy gap between the ''n''s and ''n''p orbitals of a

atom Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...

is larger than that of a

. Therefore,

fragments are in a singlet state, while

carbene In organic chemistry, a carbene is a molecule containing a neutral carbon atom with a Valence (chemistry), valence of two and two unshared valence electrons. The general formula is or where the R represents substituents or hydrogen atoms. Th ...

fragments are in a triplet state. So, when double bonds are formed by the interaction of these two fragments, disilenes which consist of two

units are ''trans''-bending and

s which consist of two

units are planar. The bending is even more extreme for the tin analogues of disilenes.

Synthesis

Disilenes are generally synthesized by reduction of 1,2-dihalodisilane, by retro-Diels–Alder fragmentation, by

dimerization In chemistry, dimerization is the process of joining two identical or similar molecular entities by bonds. The resulting bonds can be either strong or weak. Many symmetrical chemical species are described as dimers, even when the monomer is u ...

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 The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

names for silenes, simply change the -an- infix of the parent to -en-. For example, - is the silane ''disilANe''. The name of = is therefore ''disilENe''. In higher silenes, where

isomers In chemistry, isomers are molecules or polyatomic ions with identical molecular formula – that is, the same number of atoms of each element – but distinct arrangements of atoms in space. ''Isomerism'' refers to the existence or possibili ...

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

silanes In organosilicon chemistry, silanes are a diverse class of charge-neutral organic compounds with the general formula . The R substituents can be any combination of organic or inorganic groups. Most silanes contain Si-C bonds, and are discussed ...

. #Number the silicon atoms, locate and name substituent groups, locate the double bond, and name the main chain.

References

{{reflist Silicon compounds

Structure

Structure of tetramesityldisilene

Synthesis

IUPAC names

See also

References