Lead, elemental metal halides

Reactions of Trivalent Group 13 Element Halides and Metal Carbonyl Monoanions Leading to Metal Cluster Products... 

Chemical transport reactions involving the element and/or an elemental subhalide and a transition metal halide/oxyhalide. The components are heated in evacuated sealed glass tubes at 100-300 °C for a period of days to weeks. Applying a small temperature gradient (10-20 °C) leads to gas phase transport and crystallization at the cooler end of the ampoule [19]. 

Essential deviations from the theoretical predictions are observed in the saturated solutions of oxides formed by large cations such as PbO, BaO, SrO, which possess appreciable solubilities in alkali-metal halide melts. The experimentally obtained relative thermal coefficient of solubility of lead oxide in the CsCl-KCl-NaCl eutectic melts is approximately three times as high as the theoretical one. This may be explained by the distinction of the electron structure of lead from other studied metals (lead belongs to p-elements) and because of this, lead drops out of all the found regularities. Another possible reason consists in the closeness of the melt temperature (600 or 700 °C) to the melting points of lead oxide (886 °C), and the complete solubility of PbO in all the studied chloride melts is very appreciable [359]. 

The deposition of either niobium or tantalum boride by CVD is difficult to accomplish, when compared with other binary metal boride compositions. In early attempts a reaction mixture containing the metal and boron halides and Hi was used as a precursor stream. However, this reaction frequently leads to the formation of a deposit which had undergone phase separation into fractions consisting of elemental metal and boron, rather than the metal boride. The reaction only produced metal boride if very low flow rates (< 0.4 mis ) were selected [243]. Alternatively, the thermal decomposition of BBr3 and MBrs (M = Nb, Ta) has been used to deposit NbBi andTaB2 [244]. However, this reaction requires temperatures in excess 1500°C. Mixtures of tantalum pentachloride, TaCIs, and diborane, B2H6, decompose at 500-900°C to yield TaB2 films [245]. 

Until comparatively recently, the only reported stable inorganic hydrosols were primarily sols of elements such as gold, sulphur, selenium, etc. and compoimds such as silica, lead iodate, silver halides, etc. A considerable amount of attention is now being paid, however, to the preparation of mono-dispersed hydrous metal oxides, which are chemically considerably more complex than other crystalline or stoichiometrically well-defined materials and are of interest as potential catalysts. Examples include the hydrous oxides of chromium and aluminium (spheres) and copper and iron (polyhedra) with particle sizes < 1 pm. One manufacturing procedure consists of ageing aqueous... 

An alternative to cathodic deposition of the elemental metal is anodic deposition of a higher oxide (Table 2). For lead, thallium, manganese, and cobalt, this allows separation from the vast majority of metals. This concept can be extended to determination of bromide and chloride, as the respective insoluble silver halides. 

A stoichiometric amount of R4NCI will usually lead to the quantitative conversion of the acceptor chloride into the respective chloro-complex. On the other hand owing to their small DNsbCh they behave as poor solvents for most ionic compounds and also for most transition metal halides. Alkali or alkaline earth salts of the chloro-complexes cannot be formed from their solutions and chloro-complexes of most transition metal ions are also inaccessible in such media. Their use is therefore restricted to the formation of chloro-complexes of certain representative elements with large cations, such as [R4N]+. 

To these sets of primary and secondary reactions related to solvents, one has to add the eontributions of salt anion reduction, which usually forms metal halides and M AXy species (A is the main high oxidation-state element in the salt anion and X is a halide, such as chloride or fluoride). Most of the produets of aetive metal surface reactions are ionic compounds that are insoluble in the mother solution, and therefore, precipitate as surface films. It should be added to this picture that possible polymeric species can be formed, espeeially in alkyl carbonate solvents, whose reduction forms polymerizable species sueh as ethylene or propylene. Hence, the surface films formed on active metal electrodes are very complicated. They have a multilayer structure perpendicular to the metal surface, and a lateral, mosaic-type composition and morphology (i.e. containing mixtures and islands of different compounds and grains). Such a structure may induce very non-uniform current distribution upon metal deposition or dissolution processes, which leads to dendrite formation, a breakdown of the surface films, etc. These situations are demonstrated in Fig. 13.6 active metal dissolution leads to the break-and-repair of the surface films, thus forming mosaic-type structures. 

Reactions of elemental lead, which is produced in the reaction of C2H5CI and Pb-Na alloy or of Pb-Na alloy with ethyl halides in the presence of active metal alkyls, such as LiC2Hs, Grignard compounds, ethylzinc, ethylcadmium compounds, Al(C2H5)3, or ethylalu-minates, appear in the subchapters From Alloys and From Lead and Ethyl Halides or Ethyl Esters . 

All Group IV elements form tetrachlorides, MX4, which are predominantly tetrahedral and covalent. Germanium, tin and lead also form dichlorides, these becoming increasingly ionic in character as the atomic weight of the Group IV element increases and the element becomes more metallic. Carbon and silicon form catenated halides which have properties similar to their tetrahalides. 

Group-IIIB halides ( 6.5.2), through interaction with neutral metal bases, lead to acid-base adducts in which the group-IIIB elements possess higher coordination numbers than before ( 6.5.2.1) ... 

The reaction between a metal alkyl and a covalent halide of another element is sometimes made possible by the fact that transfer of the alkyl group leads to a crystalline product. An example of this type is the reaction between a sodium alkyl and a covalent halide such as SiCl4. 

Examples of the reactions of metal dianions with group 13 element halides that lead to elimination of two halide ions and formation of the formally reduced species are given in Equations (37)—(39).3S>—44... 

---