Silicon ligands transition metal complexes

The coordination sphere of transition-metal complexes can furthermore be utilized for the fixation of silicon ligands in their lowest oxidation states. Even examples of compounds containing a formally zerovalent silicon (E) are now known [41]. 

There is a large and growing field of transition metal chemistry in which silicon-containing ligands are involved. The object of this review is to provide a guide to the literature on those aspects of the subject described by the title and to deal in detail with topics not treated specifically elsewhere. Section II is concerned with complexes having Si-transition metal (M) bonds, Section III with the role of transition metal complexes in hydrosilylation, and Section IV with complexes having Si—C—M bonds. 

Early-transition-metal complexes containing silicon ligands are becoming increasingly important in organometallic chemistry1. The first group-3 silyl complex... 

One rare example of silicon-bridged early transition-metal complex of type C has been synthesized by the reaction of dimethyltitanocene with PhSiH3 [Eq. (19)].49 The structures of these products containing /r-772-HSiHPh ligands were determined by X-ray crystal structure analysis. 

Siloxides, like alkoxides, have been employed as ancillary ligands of transition metal complexes, markedly influencing the reactivity of a metal center by electronic and steric effects of the substituents at the silicon [15, 16]. 

W. Petz, Transition Metal Complexes with Derivatives of Divalent Silicon, Germanium, Tin, and Lead as Ligand", Chem. Rev. 1986, 86, 1019. 

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 chemistry of silicon, germanium, and tin transition metal compounds has been the subject of several reviews (12, 180). Optically active silyl ligands have been introduced in a transition metal complex by reaction of chiral functional organosilanes. However chiral silyl ligands containing complexes are limited to a few metal centers we shall discuss in turn iron, cobalt, platinum, and manganese complexes. 

We must however keep in mind that some of the above reactions may not be simple reactions at the silicon atom, since transition metal complexes show multicenter reactivity (metal atom, ligands) as exemplified in the chemistry of triphenylgermyl-carbene complexes of cobalt carbonyl (253). Thus, displacements of a silyl ligand may result from a multistep process and a thorough examination of these reactions has to be made. An example can be drawn from molybdenum-germanium chemistry (247). As shown in Scheme 59, germanium is displaced from complex 167 by HO with retention of configuration. Actually,... 

The stereochemistry of displacement of chiral germyl ligand in germanium transition metal complexes [which has been reviewed elsewhere (12)] will also be discussed here, since silicon and germanium resemble each other and since complementary experiments have been undertaken. 

The mechanism and stereochemistry of nucleophilic substitution at silicon, including displacement of silyl ligands in silicon-transition metal complexes. 

Interestingly, a new factor determining the predominant stereochemistry at silicon has recently been pointed out77. The LiAlH4 cleavage of hexacoordinated silyl-transition metal complexes was shown to proceed with predominant retention at silicon, regardless of the nature of the ligands around the metal atom (Table 32). The stereoselectivity was... 

A number of rc-organosilyl transition metal complexes have been studied110,115 (Table 24). There is generally a deshielding effect of 6-11 ppm for silicon atoms bonded to a ligand carbon atom directly bonded to a metal, and a very small shielding effect for... 

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