Shape four coordination

At this stage it could be useful to make a comment on the structural features of complexes 4-17. The coordination number of the metal affects strongly the shape of the calix[4]arene fragment. In the case of five-coordinate and, eventually, four-coordinate metals, the calix[4]arene moiety displays a cone, which changes to an elliptical conformation in the case of a six-coordinate metal.4,10 Such conformation changes, which appear in the solid state, are also detectable in solution, as in the... 

The two shapes associated with four-coordination are tetrahedral and square-planar. In the former only optical activity and in the latter only geometrical isomerism is generally encountered in the appropriate molecules. 

The halogen compounds are known for neutral, cationic and anionic species. These should be three, two and four coordinate, with T-shaped, angular and square planar coordination arrangements respectively. No crystallographic work has been reported on the simple species and the spectroscopic results are not always conclusive. 

Samoson et al. (141) illustrate their arguments with the 27A1 MAS NMR spectrum of sillimanite. For the signal from six-coordinated Al in sillimanite, s = 9.1 kHz (148), which means that the signal cannot be narrowed by MAS however, for four-coordinated Al, s = 5.3 kHz (line shape which compares well with theoretical calculations is indeed found in the spectrum. 

Metal complexes have characteristic shapes, depending on the metal ion s coordination number. Two-coordinate complexes, such as [Ag(NH3)2]+, are linear. Four-coordinate complexes are either tetrahedral or square planar for example, [Zn(NH3)4]2+ is tetrahedral, and [Ni(CN)4]2 is square planar. Nearly all six-coordinate complexes are octahedral. The more common coordination geometries are illustrated in Figure 20.12. Coordination geometries were first deduced by the Swiss chemist Alfred Werner, who was awarded the 1913 Nobel Prize in chemistry for his pioneering studies. 

The fundamental idea of geometrical theory of gravity starts from the fact that we can assign four coordinates to any event observed in our vicinity, for instance in Cartesian coordinates (x, y, z, t). Locally, space appears flat. However this does not prejudge of the global shape of space local observations put us in the same situation that lead people to think the earth was flat. Let us take the line element of a homogeneous 3D space which can be shown to be ... 

Figure 11.1. Schematic representation of typical silver (central atom) coordination environments with different coordination numbers and geometries (a) linear two-coordinate (b) triangular three-coordinate (c) T-shaped three-coordinate (d) square planar four-coordinate (e) tetrahedral four-coordinate (f) trigonal bipyramidal five-coordinate (g) tetragonal pyramidal five-coordinate (h) octahedral six-coordinate (i) trigonal prism six-coordinate (j) seven-coordinate (k) tetragonal prism eight-coordinate.

Elements such as B, Ga, P and Ge can substitute for Si and A1 in zeolitic frameworks. In naturally-occurring borosilicates B is usually present in trigonal coordination, but four-coordinated (tetrahedral) B is found in some minerals and in synthetic boro- and boroaluminosilicates. Boron can be incorporated into zeolitic frameworks during synthesis, provided that the concentration of aluminium species, favoured by the solid, is very low. (B,Si)-zeolites cannot be prepared from synthesis mixtures which are rich in aluminium. Protonic forms of borosilicate zeolites are less acidic than their aluminosilicate counterparts (1-4). but are active in catalyzing a variety of organic reactions, such as cracking, isomerization of xylene, dealkylation of arylbenzenes, alkylation and disproportionation of toluene and the conversion of methanol to hydrocarbons (5-11). It is now clear that the catalytic activity of borosilicates is actually due to traces of aluminium in the framework (6). However, controlled substitution of boron allows fine tuning of channel apertures and is useful for shape-selective sorption and catalysis. 

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