H-Si Systems

Compounds having M—H—Si bridges are of interest because they may be considered models for M-H-C interactions. Structural work done in this area has largely been carried out on compounds synthesized by Graham and co-workers, and is nicely summarized in a paper by Cowie and Bennett 1S4). 

The structure of W2(CO)8H2(SiPh2)2 ISfi) (Fig. 34) now appears to be the only metal carbonyl in which bona fide M-H-Si bridges are believed to exist. Again, the H atoms were not located, but there are distinct clues which indicate the presence of H bridges. The W2Si2 core of the molecule exhibits a characteristic two-short/two-long pattern of W-Si distances (2.586,2.703 A) which correspond to W—Si and W-H—Si bonds respectively (XIII). Moreover, the skewed orientation of the car- 

However, very preliminary neutron results on the related Pt(/x-H)[Si(OEt)3]2-(PMeBu h show a normal Pt(jU-H)2Pt linkage with no apparent H Si interaction160). 

Suspected Si H—M interactions were also discussed in connection with the mononuclear complexes HReCp(CO)2(SiPh3) 161), HMnCp(CO)2(SiPh3) 161,162 and HFeCp(CO)2(SiF2Me)2 163 . From an analysis of known or estimated Si — H distances, it was concluded that Si — H interactions were most likely absent in the rhenium and iron complexes. In the case of HMnCp(CO)2(SiPh3), it was originally believed that a true example of Mn—H Si interaction existed162), but a subsequent re-assessment of the problem indicates that the structural evidence is, at best, inconclusive 161.163). 

It should be pointed out that systems with M—H Si interactions investigated so far differ fundamentally from the M — H—C systems discussed in the previous section. In the former case, the main issue is whether Si H interactions are present or not, while in the latter case it is the M — H portion which is in question. 

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Despite these caveats in the application of equation B 1.5.43. one finds that it provides reasonable accuracy in many experimental situations. The SH response for the H/Si system of figure Bl.5.9 for example, is seen to obey the simple linear variation of Xs (A, )with of equation B1.5.43 rather well up to an adsorbate... 

Figure D.30. Condensed species in the B-N-Cl-H-Si system (973 K, 10 Torr, H2 = lmole, no 02, o BN, SiBi4, BN+Si3N4, Si3N4) [38]...

The apparent simplicity of the H—Si system has enabled a few realistic theoretical treatments of chemisorption on semiconductors to be made. The details of the calculations are outside the scope of this article, but it will be useful to summarize some of the principal conclusions. 

Units in SI system Si Stanton number h/Cf,pu Dimensions depend ort order of reaction. Suffixes 0 Value in bulk of phase 1 Phase 1 2 Phase 2 A Component A B Component B AB Of A in B b Bottom of column equilibrium with bulk of other phase G Gas phase / Interface value. L Liquid phase u Overall value (for height and number of transfer units) value in bulk of phase i Top of column Dimensions in in M. N, 1. T. 

Poly(hydrosilane)s are stable compounds and can be manipulated in the air only for a short period since they are oxygen sensitive. In order to study the oxidation products, a xylene solution of poly(phenylhydrosilane)(Mw = 2340, Mw/Mn = 1.72) was refluxed (140 °C) for 12 h in a system exposed to the air [15]. Only minor changes were observed by GPC analysis whereas FTIR showed characteristic absorptions due to siloxane-type structures on the polymer backbone. A detailed NMR analysis, based on H NMR, Si INEPT and H- Si HMQC spectroscopies, indicated that the oxidized material contains the units 7-10 shown in Scheme 8.2. In particular, units 7,8 and 9+10 were present in relative percentages of 27%, 54% and 19%, respectively, which mean that more than 70% of the catenated silicons were altered. It has also been reported that silyl hydroperoxides and peroxides are not found as products in the autoxidation of poly(phenylhy-drosilane) [16]. 

This section will cover aspects of monohydride terminal surface reactions that were carried out under free-radical conditions. The description will be circumscribed to the reactions with molecular oxygen and monounsaturated compounds. Mechanistic information for these reactions is scarce mainly due to the complexity of the system, and mechanistic schemes are often proposed in analogy with radical chemistry of organosilane molecules. H—Si(lll) has a band gap of about 1.1 eV while the HOMO LUMO gap in (Me3Si)3SiH is within 8-11 eV and, therefore, has very important consequences for the reactions with nucleophilic and electrophilic species where frontier orbital inter-... 

Silane o-complexes [M(p -HSiR3)(CO)5] are interesting in that here the electron density of metal is effectively delocalized over five electron-accepting carbonyl ligands so that a situation of almost negligible backdonation is modeled well. Therefore, the M" H "Si interaction in this system is very close to the 3c-2e limit thus, it is not surprising that these complexes are highly unstable. 

Figure 7.45 Equilibrium partial pressures in the Si-H-Cl system at 1 atm and Cl/H = 0.06. Reprinted, by permission, from S. A. Campbell, The Science and Engineering of Microelectronic Fabrication, p. 347. Copyright 1996 by Oxford University Press.

Velde (1963) synthesized several types of montmorillonites in the Si-Al-Mg-H O system at 2Kb pressure near 300°C. The trioctahedral forms have a substitutional series as follows ... 

Na and Ca to play equivalent roles in zeolites, as well as K, and if we consider A1 and Si as the major variables combined with K and Na in the phyllosilicates, we can adequately represent the phases in a (Ca-Na)-K-Al-Si system where H O is in excess in the fluid phase. If the system has four chemical variables and the natural assemblages are frequently found to contain four authigenic minerals, we must assume that most chemical variables are inert or extensive variables of the chemical system which controlled the crystallization of the zeolite-clay mineral containing sediments. ... 

As this book was going to press the International Uraon of Pure and Applied Chemistry recommended that (he word proton be used only when the H isotope was intended, and that ihc more general Irydron be used everywhere dsc, as in hydron donor. See Appendix t. Section 8. We have not attempted a) the last minute to change ail of these "protons to hydrons Like the SI system of units, (his change, if accepted by the world s chemsts. wtl take some lime, and the term "proton donor wil not soon disappear. 

The term agostic has now been extended lo generally include M—H—Y systems, in which Y may be B. N. Si, Cl, or F, as well as C. The structure of Ti(BH4), PMej reveals ore bidentate BH4 and two monodentate BH4 ions, each of which possesses one B—H bond attached lo titanium in a side-on, agostic manner.145... 

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