Disilanes structure

Hybrids of the type sp3 are unjustified for disilane. An important conclusion from the above hybridization statement No. 4 is concerned with the contrasting structures of the radicals SiH3 and CH3. The planar geometry of the methyl radical can readily be explained by the (bond-strengthening) sp2-hy-bridization, while the pyramidal silyl radical is thought to be stabilized (with respect to the planar arrangement) through the s-admixture to the lone electron orbital. 

The structural comparison (Fig. 3) with both the twofold tris(trimethylsilyl)methyl substituted acetylene and 1,4-benzene derivatives (Fig. 1) as well as with the literature data [6a] for hexa-kis(rm.butyl)disilane [6b] containing a SiSi bond elongated to 270 pm ( ), for the linear ( ) hexa-kis(rm.butyl)disiloxane [6c] or for di(tris(trimethylsilyl)silyl)zinc [6d] is based advantageously on a model in which the two substituent half-shells are separated along their central C3 axes by spacers of different lengths. 

The distance-dependent Pauling bond orders range from 1.00 in hexamethyldisilane with a SiSi bond of 235 pm in length (Fig. 4 standard d(l)) to 0.26 for hexakis(rert.butyl)disilane with an extremely elongated spacer distance of 270 pm between its bulky Si(C(CH3)3)3 half-shells [6b]. To rationalize the sometimes considerably weakened SiSi bonds - hexakis(rm.butyl)disilane does not dissociate into two radicals -, it has been proposed [6b,7] that additional attractive van der Waals interactions within the hydrocarbon wrapping contribute to the bonding within the respective organosilicon molecules. This assumption is further supported by the structure of hexakis(trimethylsilyl)disilane (Fig. 2), in which (presumably due to the considerable polarization Si -C5e-H5 calculated [5b]) extremely short non-bonded C(H3)-- (H3)C distances of only 352 pm are found. 

The formation of silyl radicals in the exhaustive photolysis of the silane polymers was indicated by the isolation of disilanes of general structure (HSiRR SiRR H) as shown in Table 1. These materials accumulate in the photolysate and are photostable as they absorb only weakly at the irradiation wavelength (254 nm). Longer chain silanes are presumably continuously degraded under the conditions of the exhaustive irradiation. 

Disilane and germasilane monomers have also been successfully coupled, and using trisilane monomers with different substituents on central and terminal silicons, the interesting, more structurally regular, sequence-ordered co-polymers have been prepared, as shown in Scheme 17 and in analogy to the report by West on the Wurtz-type preparation of sequence-ordered co-polymers.17... 

The much studied photochemistry of aryldisilanes carried out in earlier years has been reviewed51,52. Cleavage of the silicon-silicon bond of the disilyl moiety is always involved, but various other reactions have been observed depending on the structure of the disilane and the conditions employed. Thus cleavage to a pair of silyl radicals, path a of Scheme 15, is normally observed, and their subsequent disproportionation to a silene and silane, path b, is often observed. There is evidence that the formation of this latter pair of compounds may also occur by a concerted process directly from the photoex-cited aryldisilane (path c). Probably the most common photoreaction is a 1,3-silyl shift onto the aromatic ring to form a silatriene, 105, path d, which may proceed via radical recombination52. A very minor process, observed occasionally, is the extrusion of a silylene from the molecule (path e), as shown in Scheme 15. 

These ferrocenes containing the silicon-silicon bond are very stable under basic conditions, e.g., at reflux with 0.14 M sodium methoxide in methanol, with the exception of the l,2-(l,l -ferrocenylene)disilane with bridged structure, which in part undergoes cleavage. Cyclopentadienylpentamethyl-disilane itself is stable to the acid-catalyzed cleavage under the same conditions as used for the ferrocenes mentioned above. 

Polymeric compounds with limited Si-Si units in which the polymeric character is due to other bond systems, have been described. Another series of polymeric compounds exists, in which the Si-Si bonds themselves are responsible for the polymeric state. The extreme case, when all four valencies of a silicon atom are bonded with Si atoms, produces metallic silicon with a diamond structure. Compounds of the type (SiX2) or (SiX) result when some valencies of each silicon atom are occupied by other atoms or groups. Only polymeric compounds (SiXj,) withy from 2 to 1 exist SiX3 yields disilanes, and compounds withy between 3 and 2 yield limited chains. The possible stoichiometric compositions SiX2 and SiX are sometimes found, but a non-stoichio-metric composition is more common, which is understandable in view of the irregular framework of the Si-Si structure. The formation of regular or irregular Si-Si structures of stoichiometric or non-stoichiometric composition will depend on the procedures used in preparation. 

It has also been revealed [29] that the pore size and the affinity of a zeolite structure can be modified by chemical treatment of the zeolite structure the silane, borane, or disilane molecules are chemisorbed on the zeolite surface by reacting with the silanol groups of the zeolite [113], Polar molecules, for example, water and amines presorbed in the zeolite can be used to modify the operation of the molecular sieve and the interaction toward adsorbate molecules of the zeolite [113],... 

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