Metal alkoxides definition

Perhaps the best studied group of titanium(IV) complexes is the alkoxides. The metal alkoxides generally have received a great deal of attention because of their ease of hydrolysis and reactivity with hydroxylic molecules, and their tendency to increase the coordination number of the metal which is opposed by the steric effect of the alkyl group. These properties result in materials, the characteristics of which range from polymeric solids to volatile liquids. The definitive review of this area is that by Bradley. ... 

Definitions. Colloids are solid particles with diameters of 1 100 nanometers, A sol is a dispersion of colloidal particles in a liquid. A gel is an interconnected rigid network of sub-micrometer dimensions. A gel can be formed from an array of discrete colloidal particles (Method I) or the 3-D network can be formed from the hydrolysis and condensation of liquid meial alkoxide precursors (Methods 2 and 3). shown in Fig. 11. The metal alkoxide precursors used in Methods 2 and 3 are usually Si(OR)4 where R is CHj. C-Hj. or C3H7. The metal ions can be Si, Ti. Sn. Al, and so on,... 

The classic study of Al(OR)3 was carried out by Tishchenko [1585] at the end of nineteenth century. His dissertation, entitled On the Action of Amalgamated Aluminum on Alcohols (Aluminum Alkoxides, Their Properties and Reactions), became a great resonance for the chemistry of alkoxides (see also Chapter 1). The synthetic approaches, that he developed are still in use the purity of the samples obtained by Tishchenko, taking into account the data he reported, was definitely not worse than that of those described at present. In 1929 Meerwein and Bersin [1101], performing the acidimetric titration of Al(OR)3 solutions by solutions of alkali alkoxides, discovered the existence of bimetallic alkoxides (Meerwein salts), which play an important role in the modern chemistry of metal alkoxides. 

With the growing sophistication of X-ray crystallographic techniques, the definitive bonding modes of more and more alkoxide derivatives in the solid state are being determined. But with the rapidly advancing uses of metal alkoxides as precursors in the sol-gel process, a clearer understanding of their solution structures has become essential to understanding of the mechanistic aspects of hydrolysis and condensation as key steps in this process. 

The most commonly met regular nano-fillers are silicas and titanias. These are generally made by controlled precipitation in the presence of surfactants, to prevent agglomeration and fusion of particles. Sol-gel processes starting from metal alkoxides are often utilised. Some very fine particles produced by more conventional gas phase processes are just within the definition used for this discussion. For example, fumed silicas, with diameters of under 10 nm are available. 

Infrared spectroscopy has been utilized to verify the identity of metal alkoxides by observing bands (M-O and C-O stretching vibrations) characteristic of the bonded alkoxide group. Owing to the complex sducture and often low molecular symmetry of metal alkoxides, the assignment of various types of M-0 bands has proved to be rather difficult and the infrared technique has not generally been definitive in structural assigmnents. 

What type of compounds will be covered in this overview. Strictly, only compounds which have at least one metal-carbon bond (with the exception of metal carbides) are called organometaiiic. However, this definition was not adopted in this chapter because many families of compounds that are relevant in organometaiiic chemistry would not be included (e.g., alkoxides). By metal we mean any element from groups 1 (except hydrogen), 2, 3 (including lanthanides and actinides), 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 (except boron), 14 (except carbon and silicon), and 15 (antimony and bismuth only). Most of the available thermochemical data for all these species are freely available in a single on-line database the NIST Chemistry WebBook Unless stated otherwise, all the data included in this chapter were quoted from that reference. 

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