Carbenes and Analogues Carbenes are 6-electron carbon centres, typically  -bonded to two other groups, and with two lone electrons. These can be spin-paired (singlet) or spin-non-paired (triplet): tripletsinglet

Heavier Group 14 Analogues In the singlet state these compounds can be Lewis bases or Lewis acids. When R is a “small” group these species oligomerize. If R is large or is there is a site for intermolecular coordination to the E center (increases the coordination number up to three or four) then these species can be stabilized.

Silylene Silylene is the silicon analogue of the carbene, and is similarly unstable. They cannot be isolated by simple reduction of the dihalide: Me 2 SiCl K  2 KCl + (Me 2 Si) n + Me(Me 2 Si) m Me n = 5, 6, 7; m < 100 But photolysis or thermolysis of these species afford silylenes: c-(Me 2 Si) 6  Me 2 Si: + c-(Me 2 Si) 5 Interestingly, thermolysis either in air (300 o C) or under nitrogen (1330 o C) produces SiC, a popular industial abrasive. Silylenes are also highly reactive and thus transient species

Thermal Routes to Silylenes Elimination of a silylene from hexamethylsilirane occurs at 80 o C. It is an equilibrium reaction: The evolved silylene adds into the three-membered silirane ring to alleviate ring strain:

Formation of Si=Si - Disilenes Before coming back to silylenes we need to realize that one reaction path might be dimerization and formation of compounds containing Si=Si. Such species are not widespread – Why? Closer to the top of the group, the s and p orbital energies are closer together. Si=Si is weaker than two Si-Si bonds and the double bond is not usually kinetically stable (C=C is also weaker than two C-C, but kinetics helps) As a result silylenes tend to aggregate into rings and clusters. 6 R 2 Si:  (R 2 Si) 6 bulky ligands might protect the double bond

Formation of Si=Si The first isolable disilene (Si=Si formation) was generated in 1981 by the elimination of silane to make the disilene: Mes 2 Si(SiMe 3 ) 2 (h  )  Me 6 Si 2 + Mes 2 Si=SiMes 2 Employs bulky mesityl groups Structure is trans bent: Most are planar with Si-Si about 2.15A

Double Bonds Between E Small out of plane angle Si-Si distance 0.2 Å shorter than that of a single bond No tendency to dissociate to monomers in hydrocarbon To date approx 30 stable disilenes have been isolated and characterized (Si=Si Å, approximately planar core geometries)

Formation of Si=Si Another example:

Formation of Si=Si Photolysis: J. Am. Chem. Soc. 1993, 115, Key to formation is protection of the Si=Si with bulky groups Reduction: =

Thermal Routes to Silylenes Thermolysis of a Si=Si bond gives a silylene. Note that only due to the tremendous steric hinderance is the silylene around long enough to form a Lewis base adduct J. Am. Chem. Soc. 1997, 119(6), 1456.

Theoretical Ph 2 SiCNPh Although the compound was not structurally characterized, calculations show the silicon to be sp 3, as would make sense with a silylene-Lewis base adduct.

Reactivity of Silylenes Silylenes insert into bonds to silicon. (Note that SiMe 2 is really hexamethylsilirane heated up to ~80 o C to generate dimethylsilylene in situ) R 3 SiH + :SiMe 2  R 3 Si-SiMe 2 H R 3 SiOR + :SiMe 2  R 3 Si-SiMe 2 OR Interestingly, 1 is so stabilized by the mesityl groups (steric) that this compound can be precipitated from methanol.

Germylenes, Stanylenes, Plumbylenes As we go down the periodic table, the stability of the carbene derivatives goes up. With bulky R groups, these can be stable monomers: EX LiR  2 LiX + :ER 2 (R = CH(SiMe 3 ) 2 )

Germylenes, Stanylenes, Plumbylenes Most common way to make transient germylenes (photo- or thermoloysis): Other examples of stable monomeric species: M = Ge, Sn, Pb (via metathesis) M = Sn, Pb (via metathesis)

Germylenes, Stannylenes, Plumbylenes The two-coordinate species are considered to be sp 2 hybridized with a vacant p orbital. They tend to be colored due to the n to p transitions Three coordinate species viewed as sp 3 (T d ) and four-coordinate as dsp 3 (tbp)

Formation of Ge=Ge and Sn=Sn If the steric bulk isn’t great enough, these compounds will dimerize - Do not have planar geometry The lone pairs are in the sp 2 orbital on “E”, and it is energetically more favorable to overlap with the empty p orbital rather than promote the electron pair into the p orbital for a more classic double bond.

Double Bonds Between E out of plane angle 41° Sn-Sn distance 2.768(1) Å, which is slightly shorter than that of a single bond dissociates to (SnR 2 ) monomers in hydrocarbon

Back to Disilenes: Bonding Si=Si bond is much weaker than C=C bond consequently the HOMO LUMO gap is much lower leading to absorption in the visible

Bonding in Disilaethenes - Isomerization Isomerization of disilenes is much easier than for alkenes because the Si=Si bond is weaker Initially produced photochemically Thermally isomerized at room temp! (Note strain relief)

Reactivity Disilenes 1,2 - Additions The reactivity of disilenes has some things in common with alkenes including 1,2 addition processes, but they tend to be more reactive! This would not go without a catalyst for an alkene

Reactivity of Disilenes: 2+2 cycloadditions Unlike simple alkenes, disilaethenes readily undergo a variety of 2+2 cycloadditions

Formation of Ge=Ge and Sn=Sn An electron transfer method (reduction) can be used for formation of Ge=Ge (and Sn=Sn) bonds. (This also occurs for Si) Another method is to exchange ligands with a stable germylene. If the new ligand is less bulky, formation of the dimer occurs: ((Me 3 Si) 2 N) 2 Ge: + LiR  R 2 Ge=GeR 2 + LiN(SiMe 3 ) 2

Reactivity of Disilylenes Although the relatively weak Si=Si bond can be easily opened to form rings and disubstitutions, disilylenes never react like silylenes (insertion into  bonds: R 2 Si=SiR 2 + X 2  R 2 XSi-SiXR 2 R 2 Si=SiR 2 + HX  R 2 XSi-SiHR 2 The closest reactivity to insertion is the reaction with oxygen, which goes by a coordination pathway:

Double Bonds to Carbon From a synthetic standpoint, it is interesting to have double bond to carbon that could be exploited for C-C bond formation. The first work was done by gas phase thermolysis to produce a highly reactive species. Because of the difference in electronegativity, it is common to consider a silaethene in equilibrium with a charge separated species

Stable Silaethenes Stable silaethenes can be made by exploiting the electron-withdrawing power of the carbon substituent, and by increasing steric bulk, to prevent dimerization: This head-to-head dimerization occurs when R = Me, but not when R = CEt 3, adamantyl

Stable Silaethenes The silaethene Me 2 SiC(SiMe 3 )(SiMe t Bu 2 ) is made from an intermolecular metathesis: Si-C = A Twist of 1.6 deg.

Stable Silaethenes This is stabilized by methyl exchange in a four-centered methyl exchange: Of course, this exchange can happen with the SiMe t Bu 2 moiety as well, but is less stable:

Stable Silaethenes A similar route to silaallene:

Germaethenes Ge=C bonds can also be stabilized by lithium halide elimination: The germaethene is more stable than the silaethene due to the difference in electronegativity, but is more labile due to the weaker double bond. This gives it a broader range of reactivity.

Element- Element Double Bonds  Trans-bent configuration  The angle  increase and the H-E-H angle decreases with increasing atomic number  The ratio of the double to single bond length also increases with increasing atomic number.

Types of R 4 E 2 Structures All Sn and Pb along with some Ge and Si dissociate into type F- the inert pair effect Energetic differences between C and D are small esp. for Si Kinetic and thermodynamic stabilization through substituent design Structural types of R 4 E 2

Transition Metal-E Double Bonds Analogues of carbenes – variation of substituents and metal offers wide range of species Description of bonding

Transition Metal-E Double Bonds short tungsten-germylene bond of (16) Å vs (3) Å planar geometry around the germanium atom of the germylene ligand suggesting sp 2 hybridization Organometallics 1999, 18,

Transition Metal-E Double Bonds Description of bonding Several complexes with K structure are known – supports zwitterionic formalism (structure H)

The First Stable Cyclotrisilene The silicon analogue of cyclopropene: 3 in 65% yield together with tris(tertbutyldimethylsilyl) chlorosilane as the sole byproduct Iwamoto, J. Am. Chem. Soc. 1999, 121, 886

A Si Bow-Tie No carbon analogue to spiropentasiladiene: Dark red Thermally stable to 215C! Two rings slightly twisted from 90 Non-planar Si=Si Iwamoto, SCIENCE 290, 2000, 505.