Si—H Bond

The Si-H bond insertion reactions have been employed very effectively as a means of trapping various kinds of silylenes. (Only SiFj and SiClj do not insert into Si-H bonds.) The most popular trapping agents are silane, methylsilane, ethylsilane, and trimethylsilane. 

The insertion of SiH into Si-H bonds of SiH4 to give Si H [see equation (80)] was first employed in 1966 by Caspar and coworkers to account for the 

SiHMe insertion into Si-H bonds during the vacuum uv photolysis of CH3SiH3. From the study of disilane pyrolysis, Tebben and Ring established the insertion of SiHj into SijHg to give Si3H which was immediately confirmed by Bowrey and Purnell in 1970  

The thermal decomposition of Si2D in the presence of CH3SiH3 produced SiD4 and CH3SiH2SiD2H, which is a direct mechanistic proof for both the formation of SiD2 from Si2D and the subsequent insertion of SiD2 into Si-H bonds.  

The relative insertion rates of various silylenes, formed by the thermal decomposition of halosilanes, into the Si-H bonds of SiH3Et were also examined by Ring and coworkers  

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Make measurements of the transition wavenumbers in the rotational spectrum of silane from Figure 5.10 and hence determine the Si—H bond length. 

The properties and applications of commercially important hydride functional silanes, ie, compounds having a Si—H bond halosilanes, ie, compounds having a Si—X bond and organosilanes, ie, compounds having a Si—C bond, are discussed hereia. Compounds having Si—OSi bonds are called sdoxanes or sihcones. Those having a Si—OR bond are called siUcon esters. Sdoxanes and siUcon esters are discussed elsewhere ia the Eniyclopedia (see Silicon COMPOUNDS, SILICON ESTERS SILICON COMPOUNDS, SILICONES). 

Halogens, Hydrogen Halides, and Other Covalent Halides. Most compounds containing Si—H bonds react very rapidly with the free halogens. An explosive reaction takes place when chlorine or bromine is allowed to react with SiH at room temperature, presumably forming halogenated silane derivatives (3). At lower temperatures, the reactions are moderated considerably, for example. 

Physical Properties. The physical properties of organosilanes are determined largely by the properties of the sihcon atom (Table 2). Because sihcon is larger and less electronegative than either carbon or hydrogen, the polarity of the Si—H bond is opposite to that of the C—H bond (Table 3). This... 

Chemical Properties. Organohydrosilanes undergo a wide variety of chemical conversions. The Si—H bond of organohydrosilanes reacts with elements of most groups of the Periodic System, especially Groups 16(VIA) and 17(VIIA). There are no known reactions if the Si—H bond is replaced by stable bonds of sihcon with elements of Groups 2(IIA), 13(IIIA), and 8—10(VIII). 

Oxidation has also been cited as occurring in the cure of polymethyUiydrosiloxane [9004-73-3] (PMHS) on cellulose acetate fibers. Investigation of the cured, cross-linked siUcone shows no evidence of the Si—H bond. The same compound under an atmosphere of nitrogen does not cure and retains the Si—H bonds (99). 

Mechanistically the rate-determining step is nucleophilic attack involving the hydroxide ion and the more positive siUcon atom in the Si—H bond. This attack has been related to the Lewis acid strength of the corresponding silane, ie, to the abiUty to act as an acceptor for a given attacking base. Similar inductive and steric effects apply for acid hydrolysis of organosilanes (106). 

Whereas metal salts of carboxyUc acids cataly2e the above reactions, these are not sufftciendy basic to cleave Si—H bonds. Mercury salts of organic acids in the presence of silver perchlorate, however, do react to produce organoacyloxysdanes (111). 

The hybridic nature of the Si—H bond is utili2ed to generate C—H bonds by ionic hydrogenation according to the foUowiag general mechanism, ia which a hydride is transferred to a carbocation. 

Addition to Olefins. OrganohydrosHanes can also be prepared by addition of halosHanes and organosilanes containing multiple Si—H bonds to olefins. These reactions are catalyzed by platinum, platinum salts, peroxides, ultraviolet light, or ionizing radiation. 

We are interested in the H-H bond length, so we specify the coordinate bonding those two atoms to the AddRedundant option so that its value will be included in the printout of the optimized structure (the Si-H bond lengths will be included by default). 

E. WlBERO and E. Amberoer, Hydrides of the Elements of Main Groups I-IV, Chap. 7, pp. 462- 638, Elsevier, Amsterdam, 1971. A comprehensive review of compounds containing Si-H bonds over 700 references. 

The aim of this research was the preparation of unique silicon-functional macroreagents, particularly linear polyolefins carrying one or two Si-Cl or Si-H termini and thus to combine the excellent physical properties offered by these polyhydrocarbons with the versatility and chemical reactivity of the Si-Cl and Si-H bonds. 

F. Model Experiments on the Stability of Si-H Bonds in Carbocationic Polymerization... 

The Stability of Si-H Bonds under Carbenium Ion Polymerization Conditions... 

Evidently the Si-H bond is stable toward Me3Al under conditions similar to these prevailing during a carbocationic polymerization. 

Conclusive evidence for the stability of Si-H bond toward the benzyl carbenium ion was obtained by quantitative IR measurements. 

The Si-H bond is a strong IR absorber and this property has frequently been used in quantitative studies44. Figure 14 shows the curve constructed by the use of model compounds 7 and 9. A satisfactory linear relation exists between the absorbance of the Si-H... 

Having demonstrated that the Si-H bond is quite stable in the presence of Me3AI coinitiator and benzyl carbenium ion, further model experiments have been carried out to explore the stability of the Si-H bond in the presence of propagating carbenium ions. 

The IR calibration line shown in Fig. 36 may not be valid with the products formed in these series of experiments since in this case the densities and formula weight of the products may not be assumed the same as that of 7, which was used to construct the calibration line. Thus only H1 NMR spectroscopy has been employed to explore the stability of Si-H bonds in the presence of carbenium ions simulating propagating carbenium ions of isobutylene and styrene. 

It should be emphasized that Si-H containing compounds should be carefully handled during purification so as to avoid hydrolysis of Si-H bonds. An effective method to suppress hydrolysis of Si-H bonds is to reduce the polarity of the medium by the addition of a large amount of n-hexane before the aluminum compound is removed by washing with dilute cold HC1 solution. 

As a prelude to polymerization studies, extensive model experiments have been carried out to define conditions under which the Si-H bonds in HnSi(CH3)3 CH2CH2[Pg.30]

Head-group characterization by quantitative IR spectroscopy indicated 1.0 0.1 Si-H bond per polymer. This key data is evidence for the correctness of the proposition that PaMeSt carrying a Si-H head-group can be obtained by the use of HSi(CH3)2CH2CH29>CH2Cl/Me3Al initiating system. 

However, recently, a theoretical paper has been published, which provides interesting arguments for a conventional silylmetal hydride rather than for a 3c2e M(H)Si bond [132,133]. For the great implications of these compounds for Si —H bond activation reactions consult, e.g., on the work of Crabtree [129]. 

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