Solid transition-metal chlorides

A series of solid transition-metal chlorides can be used as positive electrodes in cells with sodium as the negative electrode. The various metal chlorides form electrochemical pairs with sodium showing different emf values (Table 1 ). 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

A major drawback connected with the symproportionation reaction is that the number of redox reactions that can take place is limited. A more recent development, which shows obvious similarities to the traditional molten salt route, but whieh avoids the limitations of symproportionations, is the solid-state technique developed by Beck in which a volatile, high-valent transition-metal chloride acts both as halide acceptor and as oxidizing agent. The synthesis of Tes " by oxidation of tellurium with WCl6 according to (4) is representative. ... 

The reaction probably proceeds through the formation of the CI3B-T1CI3 intermediate. Solid-state metathesis reactions between transition-metal chlorides and magnesium... 

Components of classic metathesis catalysts, i.e. group 5 and 6 transition-metal chlorides and organometallic cocatalysts, are more or less sensitive to moisture and air, and so should be handled in a dry, inert gas atmosphere. Although a few Schrock carbenes are commercially available, they are very sensitive to moisture and air, and so must always be handled under a strictly dried inert gas. Rh catalysts are relatively stable to air in the solid state but decompose readily in solution. 

The prime electrochemical difference between the two sodium-beta technologies is the sodium/metal-chloride positive electrode. This component contains a molten secondary electrolyte (NaAlClJ and an insoluble and electrochemically active metal-chloride phase (Fig. 40.1b). The secondary electrolyte is needed to conduct sodium ions from the primary /3"-AI2O3 electrolyte to the solid metal-chloride electrode. Cells using positive electrodes with two transition metal-chlorides, nickel and iron, have been developed. These specific metals were selected based on their insolubility in the molten NaAlCl4 secondary electrolyte. - During discharge, the solid metal-chloride is converted to the parent metal and sodium chloride crystals. The overall cell reactions for these two chemistries are as follows ... 

Tin(ll) chloride, SnCl2, stannous chloride. M.p. 247 - C. While solid (Sn plus gaseous HCl), forms hydrates (SnCl2,2H20 is tin salt) from Sn and aqueous HCl. Acts as acceptor in complexes and forms complexes with transition metals. Used as a mordant. 

The solid anhydrous halides of some of the transition metals are often intermediate in character between ionic and covalent their structures are complicated by (a) the tendency of the central metal ion to coordinate the halide ions around it, to form an essentially covalent complex, (b) the tendency of halide ions to bridge, or link, two metal ions, again tending to covalency (cf. aluminium chloride, p. 153 and iron(III) chloride, p. 394). 

Synthesis by means of volatile compounds A number of halides (especially chlorides) of the transition metals display a high volatility and in the gaseous state they are easy to mix. They can be synthesized from oxides, or scrap metals, and chlorine they are highly reactive and can be utilized for the preparation of various compounds, either as powder or a coherent solid or as coatings. Mixtures, for instance, of TiCLj + CH4 + H2 have been used to prepare ultrafine TiC powder, to deposit TiC on graphite (at 1200-1300°C), etc. 

For some applications, it is useful to put a substrate on a solid support. Linkers that can be converted directly to desired functionality ( traceless ) are particularly valuable. Andrew M. Cammidge of the University of East Anglia and A. Ganesan of the University of Southampton independently (Chem. Commun. 2004, 1914, 1916) developed the polymeric sulfonyl chloride 9. The derived phenyl sulfonates are useful partners for transition-metal mediated cross coupling. 

A third type of material is the chlorides of transition metals, such as ZnCl2 and SnCl4 (36, 37). This group of catalysts works in molten state in contrast to the solid state of the previous two groups. The corrosive nature and instability may excludes their practical application. No details are reviewed here. 

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