Properties of Silylenes

The overall chemical fate of a silylene, SiX2, in a reacting system is summarized in equation (68). This scheme is a modification of the one proposed by Atwell and Weyenberg. The silylene generated from its precursor may interact with three different types of species in a system, (i) It may dimerize or polymerize if such reactions are of high efficiency, and if the instantaneous concentration of the silylene is rather high. The dimers generally behave as reactive 

TABLE 3. Bond Angles and Bond Lengths of Silylenes in Their Ground Electronic States 

Silylenes Bond angles Si-H Bond length, nm Si-X References 

In the following sections, the chemical properties of silylenes will be described in terms of their three fundamental modes of reactions, namely, polymerization, insertion, and addition. 

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III. Spectroscopic Properties of Silylene-Lewis Base Adducts. 11... 

SPECTROSCOPIC PROPERTIES OF SILYLENE-LEWIS BASE ADDUCTS... 

The only exception to these uniform spectroscopic properties of silylene-Lewis base complexes, i.e., the hypsochromic shift on complexation, is silacarbonyl ylide 27, the absorption maximum of which is shifted to wavelengths longer than that of free dimesitylsilylene (20a) 31 an explanation of this observation is yet to come. 

Spectroscopic Properties of Silylene-Lewis Base Adducts. 

TABLE 2. Spectroscopic properties of silylene-isocycanide complexes... 

The most spectacular properties of silylenes such as 83-90 are their thermostability and their stability toward dimerization reactions leading to disilenes. 

Many of the results reviewed here suggest that a replacement of the usual alkyl or aryl substituents by silyl substituents in unsaturated silicon and germanium compounds may be rewarding. As we noted, silyl substituents do tilt the properties of silylenes, silyl radicals, and sequential BDE trends toward those in carbon chemistry. They have already been shown to stabilize disilenes with respect to dissociation to two silylenes, and this may be crucial to the further development of digermene and distannene chemistry. 

Summary Molecular orbital ab initio calculations were used to reproduce and to predict various properties of silylenes and disilenes ... 

For copolymers of structure I, for both types of side-chains, there is a striking similarity with the optical properties of the corresponding models the absorption and photoluminescence maxima of the polymers arc only 0.08-0.09 eV red-shifted relative to those of the models, as shown in Figure 16-9 (left) for the octyloxy-substituted compounds. The small shift can be readily explained by the fact that in the copolymers the chromophorcs are actually substituted by silylene units, which have a weakly electron-donating character. The shifts between absorption and luminescence maxima are exactly the same for polymers and models and the width of the emission bands is almost identical. The quantum yields are only slightly reduced in the polymers. These results confirm that the active chro-mophores are the PPV-type blocks and that the silylene unit is an efficient re-conjugation interrupter. 

Polysilanes with alkoxy groups are more light sensitive than conventional polysilanes. They degrade rapidly in the presence of light in agreement with the facile formation of silylene from dialkoxydisilanes. Properties of these polymers are currently being investigated. 

The discovery that soluble high molecular weight polysilanes may be prepared by the reductive coupling of dichlorodialkylsilanes by alkali metals (1,2) has led to considerable work on the properties of this interesting class of polymers (3,4,5). The preparation of the polymers leaves much to be desired as frequently the high polymer is only a minor product. Mechanistic studies of the reaction with a view to improving the relevant yields have been few (6). The major ones by Zeigler (7,8,9) showed that a silylene diradical was not involved in the reaction, and stressed the importance of polymer solvent interactions. 

The stability of molecules depends in the first place on limiting conditions. Small, mostly triatomic silylenes and germylenes have been synthesized successfully at high temperatures and low pressures, 718). Their reactions can be studied by warming up the frozen cocondensates with an appropriate reactant, whereas their structures are determined by matrix techniques 17,18). In addition, reactions in the gas phase or electron diffraction are valuable tools for elucidating the structures and properties of these compounds. In synthetic chemistry, adequate precursors are often used to produce intermediates which spontaneously react with trapping reagents 7). The analysis of the products is then utilized to define more accurately the structure of the intermediate. 

The ability to insert in many element-element bonds is an important property of 1 the r -p1 rearrangement of the pentamethylcyclopentadienyl ligands during the reaction is a prerequisite to show a silylene-type reactivity. From a preparative point of view it is worth mentioning that element-silicon bonds which otherwise are difficult to form are easily accessible with the help of 1. In addition, the leaving group character of the pentamethylcyclopentadienyl substituents allows further chemical transformations (vide infra). 

Besides the applications of the electrophilicity index mentioned in the review article [40], following recent applications and developments have been observed, including relationship between basicity and nucleophilicity [64], 3D-quantitative structure activity analysis [65], Quantitative Structure-Toxicity Relationship (QSTR) [66], redox potential [67,68], Woodward-Hoffmann rules [69], Michael-type reactions [70], Sn2 reactions [71], multiphilic descriptions [72], etc. Molecular systems include silylenes [73], heterocyclohexanones [74], pyrido-di-indoles [65], bipyridine [75], aromatic and heterocyclic sulfonamides [76], substituted nitrenes and phosphi-nidenes [77], first-row transition metal ions [67], triruthenium ring core structures [78], benzhydryl derivatives [79], multivalent superatoms [80], nitrobenzodifuroxan [70], dialkylpyridinium ions [81], dioxins [82], arsenosugars and thioarsenicals [83], dynamic properties of clusters and nanostructures [84], porphyrin compounds [85-87], and so on. 

E = Si) and EN(R)CH=CHNR (e.g. 2, E = Si) (R = H, Bu ) have been reported " " it was concluded that there is significant p -p -delocalisation for the latter compounds. The relationship between stability, acid-base and spin properties, nucleophilicity and electrophilicity in a series of silylenes was studied by conceptual density functional theory." ... 

The effect of substituents on the properties and structure of silylenes was analyzed in detail by Apeloig and Karni on the basis of ab initio calculations137. 

The properties of the stable silylenes raise interesting questions of chemical bonding, many of which apply also to the carbenes, 58a-f. First, why are these compounds so stable compared to ordinary silylenes (carbenes) Steric hindrance may be a factor, but it is unlikely to be a major one silylenes far more hindered than 59-63 dimerize at low temperatures, as was described in Section III.C181. 

IV. Physical Properties of Bridged Silylene and Germylene Complexes. . . 265... 

PHYSICAL PROPERTIES OF BRIDGED SILYLENE AND GERMYLENE COMPLEXES... 

The most characteristic aspect of the spectroscopic properties of bridged silylene complexes is undoubtedly the chemical shift of the bridging silicon atom in their 29Si NMR spectra. Unfortunately, the data for most of the Group 9 metal complexes and the complexes prepared before 1980 are not reported, so that almost 80% of the data are those of iron complexes. The 29Si NMR chemical shifts for bridging silylene groups are listed in Table VIII. 

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