Bonds silicon hydrides

Keywords nonclassical interactions, agostic bond, hypervalent bond, silicon, hydride... 

Silicon, unlike carbon, does notiorm a very large number of hydrides. A series of covalently bonded volatile hydrides called silanes analogous to the alkane hydrocarbons is known, with the general formula Si H2 + 2- I uf less than ten members of the series have so far been prepared. Mono- and disilanes are more readily prepared by the reaction of the corresponding silicon chloride with lithium aluminium hydride in ether ... 

The use of (TMSlsSiH with acyl selenides can also yield new C-C bond formation, as shown with the a,/l-unsaturated lactam ester (Reaction 67). The resulting ketone can be envisaged as potentially useful for the synthesis of 2-acylindole alkaloids. Both the effects of H-donating ability and steric hindrance by the silicon hydride are evident. 

Two closely related reactions, (a) and (b), illustrated by Eq. (12) (Rj = HPhj, Etj, Phj, CI3, CljPh) and (13), of silicon hydrides with transition metal complexes generate compounds with Si—M bonds with elimination of hydrogen (a) cleavage of metal-metal bonds and (b) reaction with transition metal hydrides. Reactions discussed in this section are relevant to... 

The relationship between the electronegativities of silicon, carbon, and hydrogen X(Si) < X(H) < X(C) is the reason for the polarity of the Si H bond in the sense Si+H which is reversed with respect to C-H. Therefore, silicon hydrides can easily be decomposed by hydrolysis. A fundamental conclusion from electronegativity can be formulated as follows. 

In some cases, elements having electronegativities too low to give ionic bonding with hydrogen also tend to be unreactive, so that direct combination of the elements is not feasible. In such cases, the procedure just described can be used to prepare the hydride. For example, silicon hydride, SiH4 (known as silane), can be produced by the reactions... 

The reaction of carbon-centered radicals with silicon hydrides is of great importance in chemical transformations under reducing conditions where an appropriate silane is either the reducing agent or the mediator for the formation of new bonds.23... 

Form A is a transition metal hydride while in form B the silicon hydride behaves as a two electron ligand. The electrons donated by the ligand to the transition metal happen to be a bonding pair in H-SiRj instead of a non-bonding pair in... 

Thermochemical information about neutral species can also be obtained from measurements of ions. Indeed, accurate bond dissociation energies for neutral molecules have been obtained from gas-phase ion chemistry techniques. In this section, we will summarize both the negative-ion and hydride-affinity cycles involving silicon hydrides (RsSiH) which are connected to electron affinity (EA) and ionization potential (IP) of silyl radicals, respectively [22-24]. 

The kinetic data for halogenated carbon-centred radicals with silicon hydrides are also numerous and a few examples are shown in Table 3.2. The kinetic data for perfluoroalkyl radicals were obtained by competition of the appropriate silane with the addition to an olefin [16-18]. The kinetic deuterium isotope effects (/ h/ d) on the attack of on the Si—D bond of... 

Over the last few years, silicon hydrides have been introduced as the radical mediators and the intramolecular C — C bond formation (or cyclization) by these reagents has been the subject of numerous publications. 

This value of silicon-hydride coupling constant is less than the arbitrary upper limit of bonding Si-H interactions (70-160 Hz) given on page 229 because of the rehybridization effect caused by the com-plexation of the Si-H bond to metal. Such a complexation leads to relatively large Si p character in the Si-Hterm bond, and hence the reduced 7(Si-H) for an ordinary Si-H bond. 

H MAS NMR clearly shows five signals at 0.8, 1.9, 4.4, 10.1 and 12.1 ppm. The signals at 0.8, 1.9 and 4.4 are assigned to alkyl fragments bonded to silicon atoms, residual silanol groups on the silica surface and to silicon hydride species, respectively. The two downfield signals at 10.1 and 12.1 ppm are an indication of the presence of two types of zirconium hydride surface species [111]. DQ rotor-synchronized 2D H MAS NMR was used to discriminate these zirconium hydride... 

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