Halides of metals

The majority of metallic halides are solids at ordinary temperatures and relatively few consist of finite molecules in the crystalline state. Since these compounds melt and vaporize to finite molecules or ions a comprehensive review would call for a 

Metal halides form a very large group of compounds for the following reasons. 

---

Covalent compounds, such as numerous halides of metals, give non-conducting solutions in these media. 

Strontium also is a reducing agent. It reduces oxides and halides of metals at elevated temperatures to metallic form. 

Mixtures. According to J.Cuielleron, BullFr 12, 88-9( 1945) CA 40, 4309( 1946) mixts of Na or K with halides of metals (except (hose of the alkali or alkali-earth metals) or metalloids may be exploded by a hammer blow. The same results may be obtained by substituting oxygen-containing compds for the halides... 

Halides of metals tend to be ionic unless the metal has an oxidation number greater than +2. For example, sodium chloride and copper(II) chloride are ionic compounds and have high melting points, but TiCl4 and FeCl3 sublime as molecules. 

The most favorable conditions for obtaining molecular complexes of o-hydroxyazomethines of type 856 are created by use of strong Lewis acids, for example halides of metals of Groups III-V of the Periodic Table, and aprotic organic solvents with a low dielectric constant (hydrocarbons and their halogen-substituted derivatives). An example of such synthesis is the preparation of complexes of salicylideneimines with titanium and tin tetrachlorides (4.25) [101] ... 

Other trihalides. Most of the other halides of metals of this group adopt one or more of the following structures (Table 9.17) ... 

Many halides of metals and nonmetals are good Lewis acids (see Topic C9). Such compounds are... 

Many inorganic solids such as the various halides of metals like Zn, Cd, Cu, Sn, Sb, Mn, that do not show appreciable emission at room temperatures, frequently emit rather strongly at very low temperatures of the order of — 185°C42. Uranyl salts are the best illustiations of the effect of low temperature on luminescence bands. 

The first attempts (5) to reduce metal salts with sodium at low temperatures were made by researchers working with solutions of sodium in liquid ammonia. In 1925 Kraus and Kurtz (7) showed that liquid ammonia solutions of sodium could be used to reduce halides of metals that form alloys with sodium. Operating at temperatures below the boiling point of the ammonia solutions they succeeded in reducing the halides of mercury, cadmium, zinc, tin, lead, antimony, bismuth, and thallium, and, by using an excess of sodium, concomitantly produced sodium alloys of these metals. Kraus and Kurtz postulated mechanisms for the reactions and showed that many of the alloys formed were unstable in liquid ammonia— i.e., they disproportionated into free sodium and lower sodium alloys. 

The conclusions on the anions in the ternary complexes of aromatic hydrocarbons with hydrogen halides and halides of metals drawn from the IR spectra were later confirmed by the Raman spectroscopy . The mesitylene HCl AICI3 complex displays the lines of the AlCl (350 v.s., 180m and 125w), the mesitylene HBr AlBrj complex in CH Br, those of AlBr anion (396 w, 210v.s., 114 w, 98 w). The lines of the AlCl at 183 (y ) and 126 cm" (v ) have, respectively, a triplet and a doublet splitting this indicates a d rease in the symmetry of the anion from T to (structure of 2 XY2 type ) as a result of its interaction with the cation. 

The use of different catalytic systems of both ionic [10] and ionic-coordination [13, 14] types in piperylene polymerisation has been proposed. Because of a high reaction rate of chain transfer to monomer (which grows as catalyst acidity increases) and to solvent (which drops as solvent polarity increases) in cationic polymerisation, a polymer with low MW is obtained. Some halides of metals of groups III-V were tested as catalysts of cationic piperylene polymerisation the most suitable were TiCl4 and SnC. The application of SbCl5 and InClj does not ensure an acceptable polymerisation rate and in the case of using AICI3, the insoluble polymer is obtained. 

Chemical reduction is used widely in the synthesis of coordination compounds because the oxides and halides of metals in high oxidation states are common starting materials. However, despite the large number of reductants that are available, only a limited group is used typically in preparative coordination chemistry. 

---