Coordination polyhedra, linking

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]. 

The Yb(III) ion in [Yb(acac)3 H2O 0.5CeH6] is seven coordinated and the coordination polyhedron for this complex is identical with the polyhedron found for Yb(l) in the unsolvated complex, i.e. a capped trigonal prism. Like the unsolvated complex, the hemibenzene solvated ones are also linked in pairs by hydrogen bonds. The benzene molecule occupies a centre of S3unmetry in the complex. All intermolecular contacts involving benzene carbon atoms and acac carbons or oxygens are above 3.5 A (84). 

An alternative approach is to use the bond graph to propose a coordination polyhedron for each of the cations and to generate the full structure by examining the different ways in which these polyhedra can link together to form an infinite framework. 

The complex families of silicates are best compared if the structures are described in terms of linked tetrahedra. The tetrahedral shape used is the idealised coordination polyhedron of the [Si04] unit, (Figure 7.16). Each silicon atom is linked to four oxygen atoms by tetrahedrally... 

Li+ replaces Na+. The P3cl structure, determined on a preparation in which Na Li 1 1 has two types of actinide coordination polyhedra the tricapped trigonal prism and a ten-coordinated polyhedron. These are linked to one another to form a three-dimensional polyion. 

On the kind of isomorphic substitutions significant influence has the arrangement of external electronic shell of lore It is linked with mixed kind of bonds, which never are pure ionic. Some share of covalent bond, which is always oriented in space, as the effect of Itybridized oibitals formation, significantly facilitates the coordination polyhedron formation, in accoidance with the directions of covalent bonds. The example can be the Si-0 bond (only about 60 % ionic), in which the hybridized orbitals participate. 

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