Silicon complexes

The compound rM-Fe(CO)4(SiCl3)2 was prepared via reaction of Fe3(CO)i2 and HSiCl3/ Photoelectron studies of Fe(GO)4(SiGl3)2 showed the m-isomer predominated in the gas phase. From a comparison with data for H2Fe(GO)4, it was concluded that the Fe-H orbitals were approximately 1.3 eV higher in energy than the Fe-Si orbitals.  

Mononuclear Iron Carbonyls without Hydrocarbon Ligands 

Similarly, the complex HFe(CO)3(SiPh3) Me2PCH2B(OMe)2 resulted from photolysis of Fe(GO)4 Me2PCH2- 

The reaction of a methylamine-stabilized alkoxysilyl iron complex with a cyclic stannylene gave insertion of the stannylene into the N-H bond of the methylamine (Equation (32)).  

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Stable hypervalent silicon complexes with chelating diphosphinomethanide ligands similar to II, but with hexacoordinated silicon centers are obtained from 2 and SiCl4 (Eq.(4)). 

Transition-metal chemistry is currently one of the most rapidly developing research areas. The record of investigation for compounds with metal silicon bonds is closely comparable to that for silicones it was in 1941 when Hein discovered the first metal silicon complex, followed by Wilkinson in 1956. A milestone in the development of this chemistry was Speier s discovery of the catalytic activity of nobel metal complexes in hydrosilylation reactions in 1977. Hydrosilylation is widely used in modem organic syntheses as well as in the preparation of organo functionalized silicones. Detailed investigations of the reaction mechanisms of various catalysts continue to be subject of intense research efforts. 

Kipping, F. S. 1924. Organic derivatives of silicon. Complex silicohydrocarbons [SiPh2]n. /. Chem. Soc. 125 2291-2297. 

Pentacoordinate silicon complexes can also be prepared via the reaction of diaryldichlorosilanes with carbenes 4 (R = Et, Pr R = Me). Interestingly, a 2-(trimethylsilyl)imidazolium salt (56) was formed when Me3SiI was treated with 4 (39). Unfortunately, no information regarding the crystal structures of these species is available. 

Reactions of Rhodium, Cobalt, Germanium, and Silicon Complexes... 

Hypercoordinated Silicon Complexes with Mixed Ligands 488... 

Hypercoordinated Silicon Complexes with Ligands Derived from Hydrazine 491... 

The study of compounds containing pentacoordinate silicon atoms currently represents one of the main areas of research in silicon chemistry. This is evident from the numerous reviews and proceedings published on this topic in recent years.112 Most of the pentacoordinate silicon compounds described in the literature are either salts with A5.S7-silicate anions or neutral silicon complexes with a 4+1 coordination to silicon. This review deals with a completely different class of pentacoordinate silicon compounds zwitterionic A S /-silicatcs. These molecular compounds contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom. 

The absolute configuration of the silicon complex has been established by an X-ray study and shown to be (S) starting from the (R) silane, involving retention of configuration. ... 

The hydrogen atom comes from the nucleophile and not from the complex. The iron-silicon complexes are cleaved by LiAlH with retention of configura-... 

The cobalt-silicon complex is cleaved by LiAlH with inversion of configura-... 

When L = phosphine the complex is stable to water and, in all cases, iron-silicon complexes are inert to methanol. 

The authors presented one example of 2,2,2-trifluoroacetophenone as a coupling partner with 345 (Eq. 35), suggesting that the reaction proceeds through a pentava-lent silicon complex similar to that in Scheme 46. 

Crystallogr., 1984, C40, 476 for the structure of a chiral silicon complex with 8-hydroxyquinoline ligands. 

C ls22s2p2 Si ls22s22p63s23p2). Because of the position of silicon in the third row of the Periodic Table, the chemistry of this element is influenced by the availability of empty 3d orbitals which are not greatly higher in energy than the silicon 3s and 3p orbitals. The availibility of low lying 3d orbitals to silicon and the possibility of their involvement in bond formation has been used to explain the easy formation of 5- and 6-coordinated silicon complexes, and the unexpected physical properties, stereochemistry, and chemical behaviour of a number of 4-coordinated silicon compounds. 

Another case of bridging was also reported recently in a neutral poly-silicon complex, and is discussed in the section on neutral pentacoordinate complexes (Section III.C.2). 

By this method also the zwitterionic silicates 9-15 were obtained The geometry at silicon in these compounds is TBP, like in anionic and neutral pentacoordinate silicon complexes. A typical crystal structure is shown in Figure 5 for compound 9. This structure apparently also exists in solution (CD3CN), as the 29Si chemical shift for 9 in this solvent (—122.9 ppm) compares well with the solid state CP-MAS shift of -121.0 pm28 31. 

Apart from fluoro ligands at hypervalent silicon complexes, also oxo and aza ligands are known to support penta- and hexacoordination in silicon compounds1-8. Among the various pentacoordinate oxo complexes, those with bidentate ligands (1,2-diols, aliphatic or aromatic, as well as a-hydroxycarboxylic acids) are most readily prepared, and are... 

FIGURE 6. Geometrical isomers of pentacoordinate bis(bidentate ligand)silicon complexes... 

III. PENTACOORDINATE NEUTRAL SILICON COMPLEXES A. Chelates with Nitrogen-Silicon Coordination... 

In general, pentacoordinate silicon complexes have a five-membered chelate cycle. Few exceptions are known with larger (six-membered, 102)" and smaller (four-membered 10397a, 10497b) chelate cycles. 

Nitrogen-coordinated pentacoordinate complexes have been used as stereoselective reducing agents in the preparation of erythro-(meso)- 1,2-diols from diketones and a-hydroxyketones109. The reducing agent was the (l-naphthylamino-8)trihydridosilane 92e. After formation of the dioxo chelate from the diketone (equation 32), the diol was obtained from the pentacoordinate silicon complex by reduction with LiAlILt. 29 Si NMR spectroscopy was used for the product-ratio analysis in this reaction, which was found to yield primarily the erythro diols. 

The first few pentacoordinate cationic silicon complexes (153-155) were discussed in an earlier review2. The first two were described as trivalent silicenium ions stabilized by mtermolecularcoordination183 184, while 155 is an intramolecular silicenium ion21a. Since then, a few other compounds were reported. Oxidation of the dihydrido-hexacoordinate chelate 156 by the addition of excess iodine produced 157185. The X-ray structure of 157 showed a slightly distorted TBP for the cation, with a well separated (Si—I distance 5.036 A) I8 2 anion for every two cations. The nitrogens are in the apical positions, and both Si-N distances (2.08 and 2.06 A) are longer than covalent bonds but well within the coordination distance. 

TABLE 22. 29Si chemical shifts for bis-catecholato silicon complexes with N—Si coordination (CD2G2)... 

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