Disilenes 71-71* transitions

The chemistry of the disilenes (disilaethenes) has developed very rapidly since the discovery of stable compounds. It was an obvious challenge to explore also the possibility of a n-coordination of disilenes to transition metals. According to the Dewar-Chatt-Duncason bonding model, a high stability for a disilene complex should result. 

The stable disilenes are pale yellow to orange-red in the solid state and have electronic absorption maxima in solution between 390 and 480 nm (Table I). The longest wavelength absorptions have been assigned to the ir-ir transition.28... 

The large downfield CSA for disilenes, and indeed the large isotropic chemical shift, is caused mainly by the great deshielding of one component of the tensor, o-,. Tossell and Lazzaretti propose that this deshielding results from a low-energy electronic transition between a o--bonding orbital in the molecular plane and the Si=Si 7r -orbital.45... 

From X-ray crystal structures of the products, the reactions of these stereoisomeric disilenes with episulfides, and with sulfur, were shown to proceed with retention of configuration at silicon. These findings suggest that the reaction proceeds in a concerted fashion, through intermediates or transition states involving tetracoordination for the sulfur atom being transferred (Scheme 13). Similar intermediates are believed to occur in other sulfur-transfer reactions.86... 

Reactions of disilenes to form transition metal complexes have hardly been investigated complexes are reported only for platinum. An example is shown in Eq. (35). The Pt complex 76 was identified spectroscopically and from trapping products, but it was a minor product in the reaction and crystals could not be obtained.102... 

The results in Table 3 were explained as shown in Scheme 4. From the fact that no kinetic isotope effect was observed in the reaction of phenyl-substituted disilenes with alcohols (Table 1), it is assumed that the addition reactions of alcohols to phenyltrimethyl-disilene proceed by an initial attack of the alcoholic oxygen on silicon (nucleophilic attack at silicon), followed by fast proton transfer via a four-membered transition state. As shown in Scheme 4, the regioselectivity is explained in terms of the four-membered intermediate, where stabilization of the incipient silyl anion by the phenyl group is the major factor favoring the formation of 26 over 27. It is well known that a silyl anion is stabilized by aryl group(s)443. Thus, the product 26 predominates over 27. However, it should be mentioned that steric effects also favor attack at the less hindered SiMe2 end of the disilene, thus leading to 26. 

Major advances in organometallic chemistry during the last years have been achieved in the area of silicon-metal multiple bonding and silicon with low coordination numbers. For late transition metals, new complexes have been synthesized such as silanediyl (A), silene (B), silaimine (C), disilene (D), silatrimethylenemethane (E), silacarbynes (F), cyclic silylenes (G), silacyclopentadiene (H) and metalla-sila-allenes (I) (Figure 3). 

The chapter table of contents contains subjects that were either unknown or merely distant hopes a decade ago, such as persistent silylenes, the dissociation of disilenes to silylenes and terminal silylene-transition metal complexes. The kinetics and spectroscopy of silylenes and theoretical treatments of silylene structure and reactivity have made such gigantic strides in the intervening years that they represent new vistas in our understanding. 

The color of stable disilenes in the solid state ranges from pale yellow to blue. The longest absorption maxima of acyclic disilenes in solution are assignable to the it -mi transitions and are strongly dependent on the electronic and steric effects of substituents as shown in Tables I-IV. The Amax values are 393-493 nm for tetraalkyldisilenes, 394-452 nm for dialkyldiaryldisilenes, 400-460 nm for tetraaryldisilenes, 468 183 nm... 

The transition structures for the reactions of model disilenes with haloalkanes have been located using ab initio MO calculations.128... 

Changing the substrate from ethylene to disilene (Scheme 4, M = Si) or digermene (Scheme 4, M = Ge) increases the exothermicity of the reaction (Table 41). For each H2M, M = C, Si, Ge the addition to H2Si=SiH2 is more exothermic than the addition to H2Ge=GeH2 (Table 41)204. In these reactions the complexes and transition states were not studied. 

The longest wavelength absorptions of the tetraaryldisilenes occur in the region of 400-430 nm. The three tetrasilyldisilenes 16-18 also exhibit absorption bands between 412 and 425 nm that are attributed to tt — tt transitions. Disilene 18, the sterically most heavily overcrowded compound, exhibits an additional absorption band at 480 nm. The surprising color change from yellow in the solid state to deep red in solution suggests that 18 adopts a twisted form in solution in order to reduce the steric strain39. 

In spite of the presence of usually bulky substituents at the silicon atoms, the Si—Si bond lengths are dramatically shortened and approach those of disilenes. This bond shortening is accompanied by a planar or almost planar arrangement of the substituents R1, R2 and the other silicon atom about each silicon atom. Such a geometrical arrangement resembles the bonding situation in transition metal-olefin complexes which, according to the model... 

The bonding of transition metal fragments to a silicon surface can also be described with the Dewar-Chatt-Duncanson model. There is a striking similarity between the reactivity of the Si(100) surface and that of disilene (Si2H4)97. 

Transition states and barrier heights for the silylsilylene (61)-disilene (62) isomerization via 1,2-silyl and 1,2-methyl migrations were investigated by ab initio MO calculations at the 6-31G level (equation 49). The 1,2-silyl migration of disilanylsilylene... 

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