Pyramidalization, silicon atom

Since the LUMO is n-antibonding, the kinetically controlled interaction of a donor molecule (HMPA) with the silicon leads to a decrease of multiple bonding between Cr and Si concomitant with a pyramidalization at the silicon atom. The resulting MSi bond distance and pyramidalization effect are strongly influenced by the respective substituents. 

Reaction of Cl3SiC6H2-2,4,6-Ph3 with ammonia affords the tris(amino)-silane compound (H2N)3SiC6H2-2,4,6-Ph3 in 71% yield [Eq. 9]." The X-ray crystal structure was the first for a species in which three NH2 groups were bound to one silicon atom. There is pyramidal geometry at the nitrogen atoms. The structural characterization of a series of such Si-NH2 compounds led to the conclusion that N — Si p-d 77-overlap was not significant in the Si-N bonding.99... 

Structural investigations on ( j -C5Hj)Mn(CO)2(H)SiPh3 reveal that, the hydrogen atom is located 1.55 A from the manganese atom and 1.76 A from the silicon atom. The complex has a square pyramidal geometry with the.cyclopenta-dienyl on the top... 

Thus, with an increase in the number of silicon atoms in a cyclic structure, analysis, in terms of the aromaticity concept, of the structural types and stability loses its prognostic value. A striking example of this is given by the pyramidalization of the Si atoms, typical of disilene, in hexasilaben-... 

The coordination of donors lengthens the Si=N bond and leads to pyramidalization at the silicon centre, as can be seen from the deviation of the sum of angles around the silicon atom from 360° (see examples in Table 17). Coordination of silanimines to transition metal centres also increases the silicon-nitrogen bond length to 1.66-1.69 A, which corresponds to a Si—N single bond. It has already been mentioned above that these metal compounds are best described as metallacycles349,350. 

The first synthesis of a crystalline phosphasilene 832 was achieved by a two-step reaction of t-Bu SiCI with four equivalents of SupPHLi (Sup = 2,4,6-t-Bu3CgH2) and Ph2PCl384 385 via the diphosphasila-allyl salt 833386, a remarkable conjugated sila-zr system (equation 288). The Si=P bond length in 832 is 2.094 A and the silicon atom is pyramidalized (E <= 356.7°). The 1 Jp=si of 832 is remarkably high with 203 Hz. 

The strengthening of the Si N bond is accompanied by a reduction in the XSiO angles and an increase in the nitrogen pyramidalization. These are reflected in the corresponding decrease in the deviation of the silicon atom from the plane of the equatorial oxygen atoms (Asi) and increase in the deviation of the nitrogen atom from the plane of the neighboring... 

Both experimental data and theoretical calculations show that the transition of a trigonal bipyramidal into a square pyramidal structure for compounds of penta-coordinate silicon does not require high energy consumption. Up to now the attempts of experimental separation of diastereomers of organosilicon compounds with penta-coordinate silicon atom were unsuccessful. For spirocyclic bisacetylaceto-nates, XYSi(Acac)2, containing a hexa-coordinate silicon atom, however, geometric isomerism has been observed... 

Fig. 11. Calculated pyramidalization angles at the substituted (Or) and unsubstituted (Oh) silicon atoms in mono-substituted silenes, H2Si=SiHR (6-3 IG //6-3 IG )...

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