Silyl ligands, displacements

Besides the elimination of alkali halides in the reaction between the silyl and metal reagents, several types of reactions in which other simple molecules are liberated have been widely employed. These methods, which include elimination of hydrogen, oxidative eliminations and ligand displacement reactions, are often interrelated. 

A series of ligand displacement reactions has been investigated in which a phosphine replaces a carbonyl group, the ease of reaction depending on both M and the silyl residue. Triphenyl or trialkyl phosphine can remove only one CO molecule in Mn, Re, Co, and Fe complexes, even under forced conditions,... 

Stable plumbylenes that are not strictly organo derivatives are more well known. The first of these to be prepared and fully characterized was Cp2Pb . Others, such as Pb[N(SiMe3)2]2 , Pb(PR2)2 (where R = bulky silyl ligand) , and (tmtaa) Pb(tmtaa = dibenzotetramethyltetraaza[14]annulene] are also known. Their reactions are characterized by facile ligand displacement to form inorganic plumbylenes. 

Although even in the IiAUi cleavage of the Si-Pt bonds a reductive elimination process is likely to occur, since the configuration at silicon is almost completely retained, a nucleophilic displacement of the silyl ligand is still conceivable. Deuterium labeling experiments have unambiguously demonstrated this possibility in silyl-manganese complexes (213). 

A rationalization of the stereochemistry of the displacement of silyl ligands in silicon-transition metal compounds by various reagents may be difficult to come by. However the variety of experiments which have been performed makes some facts emerge and leads to a connection with the now well documented organic chemistry of silicon. 

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,... 

An electron-transfer in the first step would give cationic intermediate 168. Formation of R3 SiX may then arise via a direct nucleophilic displacement of the silyl ligand, or through the neutral intermediate 169 by a reductive-elimination process. Cleavage reaction may also occur by competitive pathways. In the present state of knowledge, since few experiments have been done, it is difficult to pinpoint definitive mechanisms. 

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

C. Chiral Silyl-Transition Metal Complexes Nucleophilic Displacement of Silyl Ligands... 

In addition, it is possible to modify the silyl ligand via substitutions at silicon. This can be done with nucleophilic displacements at silicon (e.g. equation 28)63, or by using electrophilic reagents (e.g. equation 29)64. 

Si H M agostic interactions in silylamido complexes have been extensively studied to date. The earlier examples were prepared by halide displacement in the coordination sphere of a metal by a silylated amide, which puts severe limitations on the nature of the substituents at silicon (usually, robust methyl groups are used). More recently, a new route to p-agostic silylamides based on the direct coupling of silanes with imido ligands was discovered that allows one to trace the effect of substitution at silicon on the extent of the Si-H bond complexation (vide infra). 

The tetracoordinate silicon cation is a rather common species in solution. It may be generated by heterolytic cleavage of a bond from silicon to a reactive ligand, as a result of interaction of the silicon center with an uncharged nucleophile like amine, imine, phosphine, phosphine oxide, and amide. Since these nucleophiles are also known to be effective catalysts for many displacements at silicon including important silylation processes (86,89,235-238), the cations of tetracoordinate silicon have received attention as possible intermediates in these reactions according to Eq. (40) (78,235,239-243). 

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