Pentacoordinate silicon compounds formation

Summary The rich variety of the coordination chemistry of silicon is discussed and some theoretical issues are raised. In an attempt to understand further the underlying chemistry, some thermodynamic and kinetic parameters for the formation and substitution of pentacoordinate silicon compounds have been measured by NMR methods. Values of -31 3 kJ mol for SHand -100 10 J K mor for A5-were measured for the intramolecular coordination of a pyridine ligand to a chlorosilane moiety. A detailed kinetic analysis of a nucleophilic substitution at pentacoordinate silicon in a chelated complex revealed that substitution both with inversion and retention of configuration at silicon are taking place on the NMR time-scale. The substitution with inversion of configuration is zero order in nucleophile but a retentive route is zero order in nucleophile at low temperature but shows an increasing dependence on nucleophile at higher temperatures. These results are analysed and mechanistic hypotheses are proposed. Some tentative conclusions are drawn about the nature of reactivity in pentacoordinate silicon compounds. 

Quite recently, it was established that the formation of the pentacoordinated germanium and tin compounds EF4(CH2CH=CH2) from EF3(CH2CH=CH2) (E = Ge, Sn) by the addition of F is exothermic148. The nucleophilicity of the allylic y-carbon is much enhanced when the pentacoordinated EF4(CH2CH=CH2) is formed. These results are qualitatively similar to those found for the reaction of the corresponding silicon compound. The pentacoordinated Ge and Sn complexes have a significant Lewis acidity which allows them to form stable hexacoordinated intermediates in the course of the reaction. 

Pathways B and C describe the process that could occur to form the symmetrical silyl ketal. Formation of silyl ketal-metal complex VII could occur in one step via an associated pathway B, or in two steps via a dissociative pathway C. Pathway B could occur by attack on the pentacoordinated silyl ketal-metal complex IV by alcohol ROH, forcing the dissociation R OH leading to complex VII. The associative pathway would precede through a hexacoordinated silicon species. Hexacoordinated silicon compounds have been shown to be very reactive.36,23d The choice of the alcohol that dissociates would be determined by electronic and/or steric factors. 

In a discussion of the role of pentacoordinate phosphorus compounds in biochemistry, Zhao et al. reported the isolation of a silicon-protected pentacoordinate phosphorus compound (148) by the cyclisation of (147) formed from (145) and (146). Phosphorane (148) was then diseussed as a model for the involvement of pentacoordinate phosphorus in activating the formation of peptides from amino acids such as histidine, serine, threonine and oc-alanine but not (3- alanine. 

Alternatively, unreactive mixtures of organosilicon hydrides and carbonyl compounds react by hydride transfer from the silicon center to the carbon center when certain nucleophilic species with a high affinity for silicon are added to the mixture.76 94 This outcome likely results from the formation of valence-expanded, pentacoordinate hydrosilanide anion reaction intermediates that have stronger hydride-donating capabilities than their tetravalent precursors (Eq. 6).22,95 101... 

The enhanced reactivity of SCB-derived enol ethers is attributed to the combination of ring strain and the potential for silicon to expand its coordination number form penta- to hexacoordinate compounds. Specifically, for SCBs, the reaction with nucleophiles allows for relief of the strain energy via rehybridization of the geometry at silicon from tetrahedral to trigonal bipyramidal (tbp) upon formation of a pentacoordinate species. 

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