Silicon metal halides

An interesting variant of metal-silicon bond formation is the combination of metal halides with silyl anions. Since silyl dianions are not available, only one metal-silicon bond can be formed directly. The silylene complexes are then accessible by subsequent reaction steps [113], An example of this approach is given by the reaction of cis-bistriethylphosphaneplatinumdichloride 25 with diphenylsilylli-thium, which yields, however, only dimeric platinadisilacyclosilanes 26a-c [114]. 

It has been noted (Section II,B,1) that reactions between transition metal carbonyl anions and silicon halides often fail to produce species containing silicon-transition metal bonds, and that such failure has been ascribed to nucleophilic attack by carbonyl oxygen. It is therefore interesting that compounds containing Si—O—C—transition metal linkages have recently been isolated from such reactions [Eqs. (105) (R = Me, Ph) 183) and (106)... 

Summary Many cyclic and linear transition metal-silicon compounds have been obtained by the elimination of alkali halides. The structures of these complexes were determined by INEPT and INEPT-INADEQUATE NMR spectroscopy. 

Analogously to linear silicon halides, SigMenX does not react with Na[Co(CO>4]. However Na[Co(CO)3PPh3] and Na[Fe(CO)2cp] result in the respective transition metal-substituted silicon rings after reaction with SigMei X or 1,4-Si6Mei0X2. 

A. Reaction of a Transition-Metal Anion with a Silicon Halide. 3... 

The most important synthetic routes continue to be (1) the elimination of an alkali halide between the salt of a transition-metal anion and a silicon halide, and (2) oxidative addition and addition-elimination reactions. The present position regarding the scope and limitations of these and other routes is outlined in this section. 

In a few cases (Table I, entries 4, 18, and 19), successful syntheses have been carried out by direct reaction between a solid metal carbon-ylate and liquid silicon halide usually, however, a solvent is employed. 

Consequently, in the early work with hydridosilicon derivatives, diethyl ether was normally used in the case of very volatile products, dimethyl ether offered some practical advantages. It was tacitly assumed that a polar solvent was essential in order to dissolve, at least partly, the transition-metal carbonyl derivative. More recently, however, it has become clear that alkanes, although nonpolar, provide a very suitable reaction medium, and can be used in cases where ethers, for example, are inimical to the products (8, 32, 306, 310). The hydrocarbon, besides acting as a diluent for the silicon halide, seems to assist the separation of alkali halide from the surface of the reacting transition-metal carbonyl salt. 

Some applications of the use of substituted disilyl mercury compounds, Hg(SiXa)2 (X = Me,Cl), are shown in Table III there is an early review article (421). In most cases, the mercury compound reacts with a metal-halogen bond to give a silicon halide and mercury as byproduct, e.g. (entry 1),... 

Reference has already been made in Section II,A,3 to the harmful effects of polar solvents, in particular tetrahydrofuran, in the preparation of silicon-transition-metal compounds from a silicon halide and a transition-metal anion (61, 137, 138, 300, 305, 306, 310, 331, 337). Frequently, silicon-oxygen rather than silicon-metal compounds result, e.g. (138, 306, 336),... 

The reaction may involve the intermediate formation of a metal hydride which undergoes metathesis with the silicon halide, but no evidence for such a mechanism has been accumulated. 

Sodium Condensation. The coupling of an organic halide and a silicon halide by reaction with metallic sodium (the so-called Wurtz reaction) vras first applied by Pape in 1884. Silicon tetrachloride and isoamyl or isobutyl chloride were found to react with sodium at reflux temperatures when a little ethyl acetate was added to get the reaction started ... 

Silicon halides (especially the readily available chlorides) are probably the most synthetically useful and versatile monomeric inorganic silicon compounds. They may readily be reduced to hydrides, hydrolyzed to silanols (and subsequently to siloxanes), treated with other protic species such as alcohols, carboxylic acids, amines, and thiols to give SiA)R, Si-0C(0)R, Si NR2, and Si SR species respectively, used to make sUyl pseudohalogen derivatives by treatment with silver or alkali metal salts, for example, treated with sodium... 

The intermediate metal hydride has been isolated on occasion for Co and Mn , and Eq. (b) has actually been used to prepare silicon-metal bonds (see 5.2.3.2.2.). Inspection of Table 1 reveals the ease of reaction of Co2(CO)g compared with the other carbonyls. Normally this reaction is performed simply by condensing volatile silane onto the carbonyl in the absence of solvent and then allowing rapid reaction in the liquid phase at room temperature, but for the remaining carbonyls it is necessary to use elevated temperatures and sealed, evacuated tubes. The products are volatile and readily purified by vacuum fractionation or sublimation, but are often oxygen and moisture sensitive. The route is most efficient for RjSi derivatives of Co, Mn and Re, which are not generally obtainable by the reactions of silicon halides with metal carbonyl anions (see S.8.3.3.I.). In this way lCo(SiR,)(CO -] = Et, Phj, Clj -, (OEt)j, F/, ... 

The diruthenium and diosmium species M2(SiMe3)2(CO)g are obtainable in high yield and are reduced by sodium amalgam to [M(SiMe3XCO)4] , whose reactions with various halides provide a route to silicon-metal complexes - . Addition of silicon halides to [M(CO)4] " is successful only in the synthesis of Fe(SiH3)2(CO)4 (see 5.8.3.3.1.) and is not a route generally available for M(SiR3)2(CO)4. 

Formation of Silicon-Transition and Inner Transition-Metal Bond 239 5 8.3.3. from Silicon Halides... 

Table 1. Selected Reactions ok Metal Complex Anions with Silicon Halides...

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