Silicon compounds molecular states

H. Bock, Angew. Chem., 101, 1659-1682 (1989), Angew. Chem., Int. Ed. Engl. 28, 1627-1650 (1989), correlates in Fundamentals of Silicon Chemistry Molecular States of Silicon-containing Compounds PES and ESR/ENDOR measurement data with first and second order MO perturbation models. 

Helium l)-photoelectron spectra of silicon compounds History and achievements concerning their molecular states... 

Why then write another review on the (helium I) photoelectron spectroscopy of silicon compounds At a time of rapidly increasing computer application to various aspects of preparative chemistry, it seems worthwhile to summarize historic and more recent achievements in the rapidly progressing knowledge of silicon-containing molecules, and their molecular state properties, which are via Koopmans theorem intimately connected to quantum chemical calculations. Above all, some selected cases are well-suited to illustrate... 

This review will therefore attempt to summarize aspects of the advantageous molecular state approach to volatile silicon compounds based on selected (helium I) photoelectron spectra including their quantum chemical assignment16, and will cover some of the applications to investigations in solution as well as to the structures determined in crystals17. 

Faced with the torrent of novel silicon compounds and the sea of their molecular states, any review article can illustrate at best only some aspects of selected examples. 

Scholl et al. (32) while reporting chemical shift information have noticed that for a large number of silicon compounds the NOE varies widely and is highly sensitive to subtle changes in molecular structure. It is their conclusion that the diversity of relaxations and NOE behaviour implies differences in molecular motion in solution and liquid state structure. 

Fig. 4. Representative investigations of molecular state properties, microscopic reaction pathways, and selforganization phenomena of silicon compounds by the Frankfurt Group since 1966 (cf. Summary) [ I ].

The examples discussed in this chapter represent rather different topics organosilicon compounds, their measured molecular state properties and quantum chemical efforts to rationalize and to correlate the valuable data. All of the examples presented emphasize that molecular state considerations (Figs. 1-3) are rather useful in the fascinating world of silicon (Fig. 4). In addition, they demonstrate the increasing impact of both the constantly improved and the newly developed... 

H. Bock, B. Solouki (Helium I) Photoelectron Spectra of Silicon Compounds History and Achievements Concerning Their Molecular States in The Chemistry of Organic Silicon Compounds (Eds. Z. Rappoport, Y. Apeloig), Wiley Sons, 2001, p. 165-222 (100 references). 

Silicon is in the same family of elements as carbon in the periodic table, but silicon compounds exhibit significant differences in chemical reactivity compared to analogous carbon compounds. Similarly to carbon, silicon will conveniently bond to four substituents in its most stable state. However, silicon is more electropositive than carbon, does not form stable double bonds, and is capable of very special and useful chemical reactions that enhance its effectiveness as an adhesion promoter component. A silicon molecule that has dual reactivity can be synthesized that is, both organic and inorganic reactivities can be synthesized into a single molecule. The unique nature of silane chemistry, to have both inorganic and organic reactivities in one molecular... 

The chemistry of silicon in very low oxidation states is one of the most fascinating research areas, which can be located between molecular compounds of silicon and elemental (perhaps amorphous) silicon [190-194]. Most interesting results have recently been obtained by structural investigations of siliddes in Zintl phases. However, compounds of silicon with negative oxidation states and very low coordination numbers of 1, 2, and 3 are so far only known in the composite of a crystal lattice. 

Molecular compounds of silicon in a formally zerovalent oxidation state can be stabilized by appropriate transition-metal fragments. An entry to such polyme-tallated complexes of silicon is given by the chlorosilylene compounds 7, 11 as a starting-point. 

It has become common to classify all molecular compounds, which fulfill the above characteristics, as carbene analogs 9,13>. As a consequence, compounds of divalent silicon, germanium, tin, and lead may be regarded as carbene-like and are therefore called silylenes, germylenes, stannylenes, and plumbylenes. In contrast to carbenes they have one property in common the energetically most favorable electronic state is the singlet 1a2 found by experiments and calculations 9). 

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