Si coordination polyhedra

CH3CN, 33-11, 33 H20, 34, 35-jCH3CN, and 37jCH3CN represent nearly ideal or (more or less) distorted square pyramids, with the carbon atom Cl in the apical position. In most cases, the geometries of the Si-coordination polyhedra in the crystal are located on the Berry-pseudorotation coordinate (pivot atom Cl). 

From the results obtained for the model species 3, it is concluded that the TBP, with oxygen atoms in the axial positions, is the energetically preferred geometry of the Si coordination polyhedra of the title compounds. As the pseudorotation according to path 1 needs only a small amount of energy, package effects in the crystal (which are individual for each compound) can easily cause distortions of the TBP toward the SP. This assumption is in agreement with the results obtained in experimental studies [1]. 

Because of the presence of two identical unsymmetric bidentate ligands and two identical monodentate ligands in the chelate complexes 3-5, five stereoisomers have to be considered for these compounds (Fig. 5). As shown by the crystal structure analyses, all ctxnpounds crystallized as the (OC-6-12)-isomer (for the nomenclature system, see Ref. [14]), with slightly distorted octahedral Si-coordination polyhedra. 

Distorted trigonal-bipyramidal Si coordination polyhedra have also been reported for the related A- Si-silicates 6 7, 8, 10, 11, 12, ° 13, " and 14, with the monodentate ligands in equatorial positions. In contrast, the Si-coordination polyhedron of 9 MeOH is best described as a distorted square pyramid, the four basal positions being occupied by the two bidentate ligands. 

The Si coordination polyhedra of 16 (Figure 24.2), 17, and 18-DMF are distorted trigonal bipyramids, with the carboxylate oxygen atoms of the two bidentate ligands in the axial positions. A distorted trigonal-bipyramidal environment was also observed for the silicon atom of 19, whereas the Si coordination polyhedron of 20-CF12C12 is a square pyramid. 

NEt4]Br and silver oxalate (molar ratio 1 2 3)), was first described in 1969, and ca. 25 years later the crystal structures of the derivatives 26 and 27 were reported/ In the meantime, various tris[oxalato(2-)] silicates with different cations were describedP As shown by crystal structure analyses, the Si coordination polyhedra of 26 27 28 " (synthesis Scheme 24.2 structure Figure 24.3), and 29 " are distorted octahedra. 

MeOH). As shown by crystal structure analyses, the Si coordination polyhedra of these compounds are distorted octahedra. 

The syntheses of the first cationic tris[B-diketonato(l-)]silicon(IV) complexes, such as 48-54, were described as long ago as 1903 (synthesis of 48 Scheme 24.5). Their identities were established by elemental analyses. Eight decades later, the crystal structure of 55 was reported. In the meantime, further reports on cationic complexes of this type were published (selected publications refs. ), including the crystal structure analyses of wer-56 and 57 CH2Cl2. The Si coordination polyhedra of 55, mer-56, and 57 CH2CI2 are distorted octahedra. 

The /-coordination polyhedra of the chiral zwitterions 81-84 in the crystal were found to be distorted trigonal bipyramids, with the two carbon-linked oxygen atoms in the axial positions. This is illustrated for 81 in Fig. 10. The crystals of 81-84 are built up by pairs of (A)- and (A)-enantiomers. Selected geometric parameters for these compounds are listed in Table XI. As can be seen from these data, the axial Si-O distances [1.749(2)-1.801(4) A] are significantly longer than the equatorial ones [1.683(2)-1.7182(14) A]. The Si-C distances are 1.866(6)-1.897(2) A. 

As demonstrated by single-crystal X-ray diffraction, the /-coordination polyhedra of 85-87 are distorted trigonal bipyramids, with each of the axial positions occupied by the oxygen atoms. This is shown for compound 86 in Fig. 11. In all cases, the crystals are formed from pairs of (A)- and (A)-enantiomers. Selected geometric parameters for 85-87 are listed in Table XIII. As can be seen from the Si-O [1.8004(10)-1.829(6) A], Si-N [1.741(7)-1.764(6) A], and Si-C distances [1.867(8)-1.915(2) A], the A/02N2C frameworks of 85-87 are built up by five normal covalent bonds and do not involve a bonding system in the sense of the 4+1 coordination usually observed for pentacoordinate silicon species with Si-N bonds. 

Figure 5.16 The orthopyroxene crystal structure. The figure shows the structure projected onto (001). Oxygen coordination polyhedra [(100) projections] and metal-oxygen distances in each site are indicated (pm). Ml M2 Si. Atomic coordinates and cell parameters from Ghose (1965).

The molecular structure of (+)-25 in the crystal of the trihydrate is depicted in Fig. 10. The coordination polyhedra surrounding the silicon atoms Si and Si are distorted trigonal bipyramids (Berry distortions [8] 7.7 % and 10.6 %, respectively), in which each of the axial positions are occupied by carboxylate oxygen atoms. The molecular symmetry of the pentacyclic molecular framework of (+)-25 may be described as approximately >2 if the two ammoniomethyl groups are ignored. Thus, the absolute configurations of the two chiral silicate moieties are identical. 

The coordination polyhedra surrounding the silicon atoms Si and Si of 26 are distorted trigonal bipyramids (Berry distortions [8] 6.1 % and 10.9 %, respectively), in which each of the axial sites are occupied by carboxylate oxygen atoms. 

Structural fragment of SAPO. Ten of the 12 O anions are located by the Monte Carlo method starting from a partial map containing the four symmetry independent Si cations and using prior knowledge of their coordination polyhedra. 

There are many examples of rational synthesis. A good example is Sialon, in which A1 and oxygen were partly substituted for Si and nitrogen in silicon nitride, Si3N4. The fast Na+ ion conductor Nasicon was synthesized based on understanding the coordination preferences of cations and the nature of oxide networks formed by them. The zero-expansion ceramic CaQ5Ti2P30j2, possessing the Nasicon framework, was later synthesized based on the idea that the property of zero expansion would be exhibited by two- or three-coordination polyhedra linked in to leave substantial empty space in the network [3]. 

Network formers are cations that form coordination polyhedra in glass (like Si). 

All syntheses were carried out in acetonitrile at room temperature and compounds la, lb, 2a, 2b, and (S,S,A)-3 were isolated in good yields (78-86 %) as colorless crystalline solids. Their identities were established by elemental analyses (C, H, N), solution-state NMR experiments ( H, C, F, Si), and mass-spectrometric studies (APCl MS). In addition, all compounds were structurally characterized by single-crystal X-ray diffraction (data not given for the crystal structure of la, see [2]). The -coordination polyhedra of compounds la, lb, 2a, 2b, and (S,S,A)-3 were found to be somewhat distorted trigonal bipyramids, with the fluorine atom and carboxylate oxygen atom in the axial positions. 

FIGURE 4.35 P-cristo-balite arrangement (a) the elementary cell (b) the Si coordin-ation polyhedra for the elementary cell after Apostolescu (1982). 

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