Silicon complexes multiple bonding

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). 

Compared to the sum of covalent radii, metal-silicon single bonds are significantly shortened. This phenomenon is explained by a partial multiple bonding between the metal and silicon [62]. A comparison of several metal complexes throughout the periodic table shows that the largest effects occur with the heaviest metals. However, conclusions drawn concerning the thermodynamic stability of the respective M —Si bonds should be considered with some reservation [146], since in most cases the compared metals show neither the same coordination geometries nor the same oxidation states. 

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

The search in recent years for silicon compounds with multiple bonds or cyclic n-systems has renewed interest in siloles (66)77 and their mono- and di-anions (48 and 49), and led to the successful isolation of stable silole anions coordinated to various metal counter ions (Li+, Na+, K+)10a-c 78 - 86 and as complexes with ruthenium (e.g. 6a and 6b)10d. 

Homonuclear carbonyl dimers, palladium complexes, 8, 206 Homonuclear element-element bonds, addition to C-C multiple bonds boron-boron bonds, 10, 727 chalcogen-chalcogen additions, 10, 752 germanium-germanium bonds, 10, 747 phosphorus-phosphorus bonds, 10, 751 silicon—silicon bonds, 10, 734 tin—tin bonds, 10, 748... 

Fig. 8.3 Warren R. Roper (born in 1938) studied chemistry at the University of Canterbury in Christchurch, New Zealand, and completed his Ph.D. in 1963 under the supervision of Cuthbert J. Wilkins. He then undertook postdoctoral research with James P. Collman at the University of North Carolina at Chapel Hill in the US, and returned to New Zealand as Lecturer in Chemistry at the University of Auckland in 1966. In 1984, he was appointed Professor of Chemistry at the University of Auckland and became Research Professor of Chemistry at the same institution in 1999. His research interests are widespread with the emphasis on synthetic and structural inorganic and organometallic chemistry. Special topics have been low oxidation state platinum group metal complexes, oxidative addition reactions, migratory insertion reactions, metal-carbon multiple bonds, metallabenzenoids and more recently compounds with bonds between platinum group metals and the main group elements boron, silicon, and tin. His achievements were recognized by the Royal Society of Chemistry through the Organometallic Chemistry Award and the Centenary Lectureship. He was elected a Fellow of the Royal Society of New Zealand and of the Royal Society London, and was awarded the degree Doctor of Science (honoris causa) by the University of Canterbury in 1999 (photo by courtesy from W. R. R.)...

Little is known about the reactivities of complexes having zirconium-silicon bonds. Reaction of lb with hydrogen chloride afforded triphenylsilane (Scheme 3) [16a]. The insertion of carbon monoxide or isocyanide into a zirconium-silicon bond of lc gave silaacylzirconium complex 4 or iminosilyl-zirconium complex 6 [17b,c]. As for carbon-carbon multiple bonds, ethylene can be inserted into a zirconium-silicon bond of lh [17g],but other multiple... 

A second type of reactive metal-silicon bond involves multiple bonding, as might exist in a silylene complex, LnM=SiR2. The synthesis of isolable silylene complexes has led to the observation of new silicon-based reactivity patterns redistribution at silicon occurs via bi-molecular reactions of silylene complexes with osmium silylene complexes, reactions have been observed that mimic proposed transformations in the Direct Process. And, very recently, ruthenium silylene complexes have been reported to be catalytically active in hydrosilylation reactions. 

Heterocycles with endo- and exo Si-Si multiple bonds 04YGK94. Hydrazide-based hypercoordinate silicon compounds (Si-helates) 04MI4. Organometallic complexes of silicon analogs of azoles 03AHC(85)1. Relationship between photophysical properties and conformation of cyclic oligosi-lanes with two or more silicon atoms 02YGK762. 

A variety of catalytic bis-silylation reactions, i.e., addition of Si-Si bonds across multiple bonds, have been reported. Generally the reaction mechanism can be presented as follows (1) formation of bis(organosilyl) transition-metal complexes through activation of Si-Si bonds, (2) insertion of unsaturated organic molecules into the silicon-transition-metal bonds, and (3) reductive elimination of the silicon-element (mostly carbon) bonds giving bis-silylation products. The final step regenerates the active low-valent transition-metal complexes. Not only appropriate choice of transition metal, but also choice of suitable ligand on the transition metal is crucially important for the success of the bis-silylation reaction. In addition, substituents on the silicon atoms of disilane are also of importance. 

The hydrosilylation of carbon-heteroatom multiple bonds had received little attention until it was found in 1972 that Rh(PPh3)3Cl is an extremely effective catalyst for the hydrosilylation of carbonyl compounds. This is a new and unique reduction method since the resulting silicon-oxygen bond can easily be hydrolyzed. Other transition metal complexes including platinum, ruthenium , and rhodium also have good catalytic activity in the selective and asymmetric hydrosilylation of carbonyl compounds "". 

The bonding in the silylene addition complexes was compared with that in their carbon analogues (i.e. such as H2C h3)294. Significant differences were found, particularly in the multiple bond character of the central atom. Many of the differences between silylenes and carbenes, particularly the higher barriers to hydrogen migrations in the silylene complexes, can be understood in terms of the reluctance of silicon to rehybridize294. 

The addition of silanes across multiple bonds occurs in the presence of catalysts, mostly complexes of VIII B group elements (e.g. Co, Ni, Pd, Pt) (equation 15). Concerning the course of reaction it can be generally said that, predominantly, (a) the hydrosilylation of simple alkenes and alkynes places the silicon atom at the less substituted carbon atom and (b) via catalysts and reaction conditions a stereospecific course of reaction can be accomplished. Furthermore, as a very positive side-effect, asymmetrical hydrosilylation can be realized if chiral catalysts are employed33a 33bf 33c. For further details we recommend comprehensive review papers on this subject33a c. 

In general, low magnetic shielding of Si is typical for low-coordinated silicon atoms, as in silylenes" (e.g. 1 and 4-7 in Scheme 2), free " (e.g. 8-10" in Scheme 3) or almost free " silyl cations, in contrast with silylene adducts" (Scheme 4), Low Si nuclear magnetic shielding is typical for base-free silylene transition metal complexes " (Scheme 5), and also for numerous compormds, where silicon is involved in multiple bonding with other elements (Scheme 6). 

In the last few years the design and use of various disilane compounds has gained importance because of the reactivity of the Si-Si bond and the large potential for organic synthesis involved with it. Many publications offer us numerous examples of possible reactions at the silicon-silicon bond such as addition reactions with C-C double bonds or C-C triple bonds [1, 2], addition reactions with C-element multiple bonds (e.g. aldehydes, quinones, isocyanides) [3-5] or metathesis [6, 7] and cross-metathesis [8]. In the most cases the existence of a catalyst (palladium, platinum or nickel complexes) for activation of the silicon-silicon a bond is indispensable for a successful transformation [9-11]. 

Silica is one of the most common elements, and is present in many natural waters. It is found in waters in the form of (a) reactive silica, (b) colloidal silica, and (c) suspended particles (sand). Reactive silica is called silicon dioxide, and in this form is generally not ionised at normal pH levels of water [33,37]. Colloidal silica is either polymerised silicon with multiple units of silicon dioxide, or silicon that has formed loose bonds with organic compounds or other complex inorganic compounds such as calcium oxide and... 

The insertions of olefins into metal-silyl complexes is an important step in the hydrosi-lylation of olefins, and the insertions of olefins and alkynes into metal-boron bonds is likely to be part of the mechanism of the diborations and sUaborations of substrates containing C-C multiple bonds. Other reactions, such as the dehydrogenative sUylation of olefins can also involve this step. Several studies imply that the rhodium-catalyzed hydrosilylations of olefins occur by insertion of olefins into rhodium-silicon bonds, while side products from palladium- and platinum-catalyzed hydrosilylations are thought to form by insertion of olefins into the metal-sihcon bonds. In particular, vinylsilanes are thought to form by a sequence involving olefin insertion into the metal-silicon bond, followed by p-hydrogen elimination (Chapter 10) to form the metal-hydride and vinylsilane products. 

The orbitals involved in the formation of a metal-silicon multiple bond are shown in Figure 13.6. These orbital interactions are the same as those involved in the formation of a metal-carbon double bond in a carbene complex. However, back-donation from the metal d-orbital into the silicon p-orbital is much weaker in a metal-silylene complex than it is in a carbene complex. Therefore, the silicon remains Lewis acidic, and the silylene complexes are often stabilized by Lewis bases. 

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