Divalent silicon compounds

Divalent silicon compounds (silylenes), one of the most interesting class of low-coordinated silicon compounds, had been known as highly reactive, short-lived transient species that resembled the carbon analogues (carbenes), until the recent... 

Among the few stable divalent silicon compounds that have been isolated, there are the tetracoordinate (iff-tbp) Si[(PMe2)2C(SiMe3)]2144 (8-XX) and 2-coordinate... 

Silylene (SiHa) and its derivatives with small organic substituents R represent highly reactive species, which are key intermediates in many thermal and photochemical reactions. They have been characterized by matrix and dilute gas phase studies [1]. The last two decades have seen great progress in the kinetic and/or thermodynamic stabilization of divalent silicon compounds and in the isolation of species persistent imder ordinary conditions (Fig. 1) [2 - 6]. 

Different Reactivities of Divalent Silicon Compounds Towards Metallocene Derivatives of Molybdenum and Tungsten... 

Keywords Divalent Silicon Compounds / Molybdenum Compounds / Silylene Complex / Tungsten Compounds / X-Ray Crystal Structures... 

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). 

Some years ago we were successful in our attempts to synthesize a Jt-complex with divalent silicon as the central atom. Starting from dihalogeno(pentamethylcyclopentadienyl)silanes, we have been able to prepare decamethylsilicocene (1) by reductive elimination processes [1]. Characteristic data concerning the synthesis, structure, and bonding of 1 have been published elsewhere together with preliminary results concerning the chemistry of this compound [2]. Here we describe some further progress in this field. 

The soluble divalent SiF2 compound is in turn transformed by disproportionation into SiF6 and elemental amorphous silicon. This mechanism is responsible for the effective dissolution valence of Si, which was found to be equal to 2 in the range of potential between 0 and +0.4V/SCE. 

The reaction of 82 with protic substrates HX can follow different pathways, as depicted in Scheme 23. In most of the cases studied so far, simple oxidative addition takes place leading to compounds of the type (Me5C5)2Si(H)X190. The reaction of 82 with tetrafluoroboric acid leads to elimination of pentamethylcyclopentadiene and to the formation of a cyclotetrasilane derivative, via several intermediates with divalent silicon from which the dimeric pentamethylcyclopentadienyl(fluoro)silylene was proved by Si NMR spectroscopy191,192. In contrast, an HF-oxidation product is formed in the reaction of 82... 

Silicon in the lower oxidation states is discussed by Burger673. Silicon compounds in the divalent state also exist with formation of Si—Si bonds. In the carbon group the tendency to divalent state increases with higher atomic numbers while the tendency to tetravalence declines. Since silicon atoms are relatively small, the tetra valent state is strictly preferred. 

Divalent silicon may be present in different types of compounds (A—D). Of these the ions SiX2+ (C) and SiX2 (D) have been detected in mass spectrometry. 

Photochemical behavior of monosilanes has been investigated by mercury-sensitized photolysis (15-19), flash photolysis (20, 21), vacuum ultraviolet photolysis (22-27), and matrix photolysis (28-30). The first examples of the photolysis of permethylated polysilanes were published in 1970 (14). All of the cyclic and acyclic permethylpolysilanes with the exception of hexamethyldisilane readily undergo photolysis on irradiation with ultraviolet light to give shorter chain compounds with the concurrent generation of the divalent silicon intermediate, dimethylsilylene (8). 

Silylenes Divalent, dicoordinate silicon compounds, are the silicon coimterparts to the carbenes well known in organic chemistry. Since silylenes are frequent intermediates in both thermal and photochemical reactions, their importance in organosilicon chemistry is great [1]. There is recent evidence that even the direct reaction of methyl chloride with silicon, the foundation stone of the worldwide silicone industry, may proceed through the formation of silylene intermediates [2]. 

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