Trivalent Silicon

Trivalent silicon has so far been established in species without charge (A) and negative charge (B) cations have not been detected despite great effort directed towards their preparation or even detection as intermediates. 

Type (D) structures with sp2 hybridized silicon corresponding to CH20 or COCl2 have not yet been isolated, but are supported by spectroscopic investiga- 

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When the counterion is complex, for example metal-halogen anions such as BF4-, the most electronegative portion of the counterion becomes attached to the silicon center. Because of this attachment, it is natural to consider the intermediacy of a silicenium cation (silylium or silylenium ion) intermediate in such reactions (Eq. 4). Bond energies derived from electron impact studies indicate that Eq. 4 is exothermic in the gas phase by about 8 kcal/mol.26,29 There seems little doubt that trivalent silicon-centered cationic species do exist in the gas phase30,31 or that processes similar to that shown in Eq. 4 do occur there.32,33... 

Uncertainties in understanding the exact mechanistic details of these reactions are sure to stimulate continued work to define the nature of trivalent silicon cations in ionic reductions by organosilicon hydrides. 

Open-chain alkanes, alkyl halide reduction, 29-31 Organosilicon hydrides bond strengths, 5-6 hypervalent silicon species, 9-11 ionic hydrogenation, 5 trivalent silicon species, 7-9 Orthoesters, reduction of, 97-99 Oxime reduction, 102... 

Trivalent silicon species, organosilicon hydride reductions, 7-9... 

With the compound (2-Me2NCH2C6H4)PhSi(H)(03SCF3) 786, it was demonstrated that more than one built-in ligand is needed for the stabilization of trivalent silicon cations as, in this structure, the triflate anion is involved in the coordination to the silicon atoms.810 The related Si-O bond distance of 1.951(1) A is longer than the standard Si-O bond length (approx. 1.6 A) and therefore compound 786 might be best described as contact ion pairs (Figure 5). 

A general comment on the use of the empirical correlation between Si and Sn NMR (and likewise on C/ Si or Sn/ Pb NMR) chemical shifts is in order. The basis for this correlation is that the paramagnetic term Op dominates the chemical shift. According to Ramsay s theory, Op is proportional to the reciprocal energy difference h.E between the magnetically active orbitals and proportional to the expectation value for the electron radii (r )np- Thus, a linear correlation between the 5 Si and 8 Sn implies that the ratio of both determining factors of Op is constant for the all compounds of interest. In particular, it is not clear, however, if the ratio for tetravalent silicon and tin compounds is the same as for trivalent silicon and tin compounds. Therefore, the extension of a correlation based exclusively on the... 

Configurational stability of Ph(/-Pr)2SiLi has been estimated by means of H NMR [Eq. (9)] (24). The two methyl groups in each isopropyl group are diastereotopic and anisochronous at room temperature in a wide range of solvents. The nonequivalence is observed up to I85°C. Since rotation around the Si-C bonds appears to be fast on the NMR time scale, the nonequivalence of the isopropyl methyl groups in the anion is evidence for slow inversion about silicon. This result indicates that unimolecular atomic inversion about silicon must be slow on the NMR time scale, and a lower limit to inversion about the trivalent silicon can be set at about 24 kcal/mol, which is consistent with theoretical studies (see Section VIII). 

Figure 6.21. (a) Scheme of electronic transitions (b) general shapes of IR absorption for surface with n-type conduction (c) absorption of defects of trivalent silicon in Si02-Si. 

Attack on oxiranes by trivalent phosphorus (64HC(19-l)43l) provides a method of deoxygenation to alkenes with inversion (c/. Section 5.05.3.4.3(hY)) and this makes possible the interconversion of (Z)- and (f)-alkenes (Scheme 58) (B-74MI50505). Silicon nucleophiles behave analogously (76JA1265, 76S199). 

A trivalent hard chromium bath has recently been described . The bath contains potassium formate as a complexing agent, and thicknesses in excess of 20 m can be deposited. Hardnesses of up to l650Hy can be obtained by heat treatment at 700°C. The deposits contain 1.6-4.8% carbon, and the bath is suitable for the deposition of composite deposits containing diamond or silicon carbide powder. 

The initial photochemical step in almost all of the reactions described in this chapter is formation of either trivalent radicals of the type R3E-, or else the divalent analogues of carbenes, R2E . Such species are obviously very reactive, and are only observed as intermediates or in experiments in the presence of trapping agents. The relative stability of the intermediates depends greatly on the nature of the substituents R, and this can influence the type of reaction products ultimately formed. Where appropriate, comparisons with the behaviour of the analogous silicon species are made. 

The other major approach toward overcoming the "alkyl transfer" difficulty of the Abramov reaction involves the use of silyl esters of the trivalent phosphorus acids. Unlike carbon, silicon does not have the stereochemical restraints associated with ordinary alkyl groups for intramolecular transfer.211 The preparation of mixed alkyl—silyl esters of trivalent phosphorus acids paved the way for the Abramov reaction to be of general utility.204 208 212 An example is shown in Equation 3.14. 

With the potentially bis-chelating 2,6-bis(/V,V-dimethylaminomethyl)phcnyl ligand, which can be attached to silicon via its lithium salt (Figure 4), Corriu et al. have prepared a number of trivalent silyl cations as their chloride salts 787-790 (Scheme 103).811... 

The destabilizing effect of a silyl group compared with an alkyl group in trivalent carbocations was explained by the weaker hyperconjugation of the Si-R a-bond (R = alkyl) relative to a C-R cr-bond (R = H or alkyl) and by electrostatic repulsion between the adjacent positively charged cationic carbon and the electropositive silicon (10). 

The selectivity of glass for alkah metal ions is connected with the presence of oxides of trivalent metals in the glass structure. Zachariasen [450] states that silicate glass has a random cross-linking, where each silicon atom lies in the centre of a tetrahedron formed of oxygen atoms (see planar scheme (6.5.5)). 

The presently known silicon chemical shift range is 990 ppm. This includes the Dsd form of decamethylsilicocene 28 (5 Si = —423 (solid state)), which is the most shielded resonance reported to date and the alkyl-substituted silylene 45, which presently defines the high-frequency end of the spectrum at 5 Si = 567. Most silicon chemical shifts occur, however, in a much smaller range from 5 Si = +50 to —190. This includes hexa-, penta- and tetracoordinated silicon compounds and for trivalent, positively charged silicon a significant low-field shift compared to comparable tetravalent silicon species is expected. 

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