Four-coordinate species shape

Zeolites are a subclass of microporous materials in which the crystalline inorganic framework is composed of four-coordinated species interconnected by two-coordinated species. Traditionally these materials are aluminosilicates however, many different compositions have been synthesized. The templates used in the synthesis of microporous materials are typically small ionic or neutral molecular species. The function of the template in the synthesis of microporous materials is little understood, and there are at least four different modes by which an additive can operate in a zeolite synthesis a) It may act as a space filler occupying the voids in the structure, thereby energetically stabilizing less dense inorganic framework b) the additive may control the equilibria in the synthesis mixture, such as solution pH or complexation equilibria c) it may preorganize the solution species to favor the nucleation of a specific structure d) it may act as a true template determining the size and the shape of the voids in the structure. 

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. 

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. 

Complexes of zero-valent palladium (d ) are tetrahedral if the coordination number is four, trigonal if the coordination number is three, and hnear if the coordination number is two. For Pd (d ), the geometry is square planar, unlike nF, which is normally octahedral, but like PF. With a very bulky ligand, 14-electron three-coordinate T-shaped species can be stabilized and structurally characterized. For example, the complex Pd(Ar)l[P(/-Bu)3], where Ar = 2,4-xylyl, can be isolated and the aryl group is opposite the open coordination site. For the few Pd (d ) complexes that have been structurally characterized, octahedral geometry is found. 

Although a number of factors influence the number of ligands bonded to a metal and the shapes of the resulting species, in some cases we can predict which structure is favored from the electronic structure of the complex. For example, two four-coordinate structures are possible, tetrahedral and square planar. Some metals, such as Pt(II), form almost exclusively square-planar complexes. Others, such as Ni(II) and Cu(II), exhibit both structures, depending on the ligands. Subtle differences in electronic structure, described later in this chapter, help to explain these differences. 

Theoretical studies have also been made on nickelacyclopentane. In practice the product of decomposition of such a compound, [L Ni], depends on its coordination number, being, respectively, butene, cyclobutane, and ethene for (n + 2) = 3, 4, and 5. The theoretical studies show that for planar four- and three-coordinate systems viz. cis-[L2Ni] and [LNi]), the reductive elimination of cyclobutane is symmetry allowed, while the production of ethene is not. The reverse is true for tetrahedral [L2Ni]. Five-coordinate species, [LsNi], can give either cyclobutane or cyclobutane and ethene depending on their shape. 

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