Disilene
Encyclopedia
Disilenes are compound
s containing a silicon
–silicon double bond
and are considered to be heavier analogues of alkene
s. They are sometimes also called disilaalkenes.
.
or other reactions, so their life is very short.
To prevent this polymerization
and other reactions, bulky substituent
s are used to stabilize disilenes effectively for long-term survival in dilute solution
and even in crystal
s.
Stable disilenes are generally yellow- or orange-colored crystalline compound
s.
The Si=Si double bond
lengths of disilenes vary between 2.14 and 2.29 Å and are nearly 5 to 10% shorter than the Si-Si single bond
lengths of corresponding disilanes.
This rate of bond shortening is less than ca 13% in carbon
compound
s, but is short enough for true double bond
characteristics.
A further peculiarity of disilenes is the trans-bending of the substituent
s, which is never observed in alkene
s.
The trans-bent angles of disilenes between the R2Si planes and the Si=Si vector range from 0 to 33.8 deg.
This is rationalized by the stability of the corresponding silylene
fragments.
The valence
orbital
s of silicon
are 3s
and 3p
, while those of carbon
are 2s
and 2p
, so the energy gap between the ns
and np
orbital
s of a silicon
atom
is larger than that of a carbon
atom
.
Therefore, silylene
fragments are in a singlet
state, while carbene
fragments are in a triplet state.
So, when double bond
s are formed by the interaction of these two fragments, disilenes which consist of two silylene
units are trans-bending and alkene
s which consist of two carbene
units are planar.
of 1,2-dihalodisilane, by retro-Diels–Alder fragmentation, by dimerization of silylene
s, by photofragmentation of cyclopolysilanes, or by rearrangement of silylsilylenes.
In one study a disilene is prepared by an intramolecular coupling of a 1,1-dibromosilane with potassium graphite. The silicon double bond in the resulting compound has a bond length
of 227 picometer (second largest ever found) with trans-bent angles 33° and 31° (by X-ray diffraction).
In addition to this the substituents around the Si-Si bond are twisted by 43°. The disilene isomerizes to a tetracyclic compound by heating at 110°C in xylene
thereby releasing its strain energy
.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s containing a silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
–silicon double bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
and are considered to be heavier analogues of alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s. They are sometimes also called disilaalkenes.
History
The first transient disilene was synthesized in 1972 by D. N. Roark and Garry J. D. Peddle. The first thermally stable disilene, tetramesityldisilene, was isolated in 1981 by Robert WestRobert West
Robert C. West is E. G. Rochow Professor of Chemistry Emeritus at the University of Wisconsin–Madison; Director of the Organosilicon Research Center, University of Wisconsin–Madison 1999–present; President, Silatronix, Inc...
.
Properties
Simple disilenes are very reactive species and easily undergo polymerizationPolymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
or other reactions, so their life is very short.
To prevent this polymerization
Polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
and other reactions, bulky substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...
s are used to stabilize disilenes effectively for long-term survival in dilute solution
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
and even in crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
s.
Stable disilenes are generally yellow- or orange-colored crystalline compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s.
The Si=Si double bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
lengths of disilenes vary between 2.14 and 2.29 Å and are nearly 5 to 10% shorter than the Si-Si single bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
lengths of corresponding disilanes.
This rate of bond shortening is less than ca 13% in carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
s, but is short enough for true double bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
characteristics.
A further peculiarity of disilenes is the trans-bending of the substituent
Substituent
In organic chemistry and biochemistry, a substituent is an atom or group of atoms substituted in place of a hydrogen atom on the parent chain of a hydrocarbon...
s, which is never observed in alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s.
The trans-bent angles of disilenes between the R2Si planes and the Si=Si vector range from 0 to 33.8 deg.
This is rationalized by the stability of the corresponding silylene
Silylene
Silylenes are chemical compounds containing a divalent silicon atom without any electrical charge. Both dicoordinate and tricoordinate silylenes are reported in the literature. They are considered to be heavier analogues of carbene. In earlier times, they were called silene, but this is a mistake,...
fragments.
The valence
Valence (chemistry)
In chemistry, valence, also known as valency or valence number, is a measure of the number of bonds formed by an atom of a given element. "Valence" can be defined as the number of valence bonds...
orbital
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
s of silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
are 3s
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
and 3p
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
, while those of carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
are 2s
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
and 2p
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
, so the energy gap between the ns
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
and np
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
orbital
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
s of a silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
is larger than that of a carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
.
Therefore, silylene
Silylene
Silylenes are chemical compounds containing a divalent silicon atom without any electrical charge. Both dicoordinate and tricoordinate silylenes are reported in the literature. They are considered to be heavier analogues of carbene. In earlier times, they were called silene, but this is a mistake,...
fragments are in a singlet
Diradical
A diradical in organic chemistry is a molecular species with two electrons occupying two degenerate molecular orbitals . They are known by their higher reactivities and shorter lifetimes. In a broader definition diradicals are even-electron molecules that have one bond less than the number...
state, while carbene
Carbene
In chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is RR'C:, but the carbon can instead be double-bonded to one group. The term "carbene" may also merely refer to the compound H2C:, also called...
fragments are in a triplet state.
So, when double bond
Covalent bond
A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....
s are formed by the interaction of these two fragments, disilenes which consist of two silylene
Silylene
Silylenes are chemical compounds containing a divalent silicon atom without any electrical charge. Both dicoordinate and tricoordinate silylenes are reported in the literature. They are considered to be heavier analogues of carbene. In earlier times, they were called silene, but this is a mistake,...
units are trans-bending and alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
s which consist of two carbene
Carbene
In chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is RR'C:, but the carbon can instead be double-bonded to one group. The term "carbene" may also merely refer to the compound H2C:, also called...
units are planar.
Synthesis
Disilenes are generally synthesized by reductionRedox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
of 1,2-dihalodisilane, by retro-Diels–Alder fragmentation, by dimerization of silylene
Silylene
Silylenes are chemical compounds containing a divalent silicon atom without any electrical charge. Both dicoordinate and tricoordinate silylenes are reported in the literature. They are considered to be heavier analogues of carbene. In earlier times, they were called silene, but this is a mistake,...
s, by photofragmentation of cyclopolysilanes, or by rearrangement of silylsilylenes.
In one study a disilene is prepared by an intramolecular coupling of a 1,1-dibromosilane with potassium graphite. The silicon double bond in the resulting compound has a bond length
Bond length
- Explanation :Bond length is related to bond order, when more electrons participate in bond formation the bond will get shorter. Bond length is also inversely related to bond strength and the bond dissociation energy, as a stronger bond will be shorter...
of 227 picometer (second largest ever found) with trans-bent angles 33° and 31° (by X-ray diffraction).
In addition to this the substituents around the Si-Si bond are twisted by 43°. The disilene isomerizes to a tetracyclic compound by heating at 110°C in xylene
Xylene
Xylene encompasses three isomers of dimethylbenzene. The isomers are distinguished by the designations ortho- , meta- , and para- , which specify to which carbon atoms the two methyl groups are attached...
thereby releasing its strain energy
Strain energy
In a molecule, strain energy is released when the constituent atoms are allowed to rearrange themselves in a chemical reaction or a change of chemical conformation in a way that:* angle strain,* torsional strain,* ring strain and/or steric strain,...
.
See also
- DisilaneDisilaneDisilane is a chemical compound with chemical formula Si2H6 that was identified in 1902 by Henri Moissan and . Moissan and Smiles reported disilane as being among the products formed by the action of dilute acids on metal silicides...
- DisilyneDisilyneA disilyne is a chemical compound that contains a formal silicon - silicon triple bond and as such is formulated R2Si2 and is the silicon analogue of an alkyne. The hypothetical parent hydride, Si2H2 does not actually exist, all attempts to synthesize it results in a diradical disilene, with the...
- Organosilicon chemistryOrganosiliconOrganosilicon compounds are organic compounds containing carbon silicon bonds. Organosilicon chemistry is the corresponding science exploring their properties and reactivity.Like carbon, the organically bound silicon is tetravalent and tetrahedral...
- Double bond ruleDouble bond ruleThe double bond rule states that chemical elements with a principal quantum numbergreater than 2 do not form multiple bonds with themselves or with other elements...