Two-coordinate species

The reactions of bare metal ions with neutral molecules increases the coordination number but may produce complex ions that would be considered under coordinated in the condensed phase. Two coordinate species (where L is a neutral ligand) are common in the gas phase. The reduction in the reactivity in the series M+ —> [ML]+ —> [MLaf is quite common resulting in the two coordinate species showing low reactivity. 

Two-coordinated species are of two types discrete monomers, usually linear, and X—M—X linkages as part of infinite chains. 

From the evidence described above concerning the uncommon behavior of radical reactions in hydrocarbon oxidation at a titanium center inside a zeolite, the possibility that peroxo radicals are also involved in oxidation reactions of other compounds should be considered. In contrast to the mechanism discussed above for the hydrocarbons, in this case both the peroxo and the reactant molecule would be coordinated at the Tilv center, and the reaction would therefore take place between two coordinated species. 

Different geometrical arrangements of the atoms attached to the central atom are possible for all coordination numbers greater than one. Thus, two-coordinate species may involve a linear or a bent disposition of the ligands and central atom. Similarly, three-coordinate species may be trigonal planar or trigonal pyramidal, and four-coordinate species may be... 

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. 

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