Hafnium, elemental halides

Element 104. (eka-hafnium) is predicted to resemble its homolc hafnium (element 72) in its chemical properties. It is expected to be predominantly tetra-positive, both in aqueous solution and in its solid compounds, although it should exhibit sohd halides and perhaps aqueous ions of the +2 and +3 oxidation state as well. 

In contrast to the zirconium compounds the chromium halides are less active than pure Cr(allyl)3 Cr (allyl) Cl2 seems to be completely inactive. The only other element known in which halide substitution of the corresponding allyl compound results in activation is hafnium, suggesting the effect may be confined to group IVa. 

These two elements have very similar chemistries, though not so nearly identical as in the case of zirconium and hafnium. They have very little cationic behavior, but they form many complexes in oxidation states II, III, IV, and V. In oxidation states II and III M—M bonds are fairly common and in addition there are numerous compounds in lower oxidation states where metal atom clusters exist. An overview of oxidation states and stereochemistry (excluding the cluster compounds) is presented in Table 18-B-l. In discussing these elements it will be convenient to discuss some aspects (e.g., oxygen compounds, halides, and clusters) as classes without regard to oxidation state, while the complexes are more conveniently treated according to oxidation state. 

We have found tantalum to be especially suitable for synthetic reactions and equilibrations involving elements and their reduced halides which lie to the left of group V in the periodic table, namely the alkali metals and alkaline earth metals scandium, yttrium, and the lanthanides titanium, zirconium, hafnium, thorium, and uranium. Tantalum and niobium are also uniquely suitable containers for the syntheses of their own lowest halides, for example, Ta6Br14 3 and CsNb6In.4 Tantalum containers have been extensively employed for the synthesis of halides, but reduced compounds of some other nonmetals, some oxides, for example, perhaps can be handled as well. 

Carbides of metals and other elements have been produced by this approach, including carbides of boron, silicon, titanium, zirconium, hafnium, vanadium, niobium, molybdenum, tungsten, tantalnm, and thorium (Funke, Klementiev, Kosukhin, 1969 Sheppard Wilson, 1972 MacKinnon Wickens, 1973 Chase, 1974 Steiger Wilson, 1974 Swaney, 1974 MacKinnon Renben, 1975). The produced caibide particles are very small their diameter is usually about 20-200 mu. Halides are mixed with hydrocarbons, usually in a ratio H2 Me = 2-30. The gas mixture is heated up in plasma to temperatures of 1300-4000 K time of synthesis exceeds 50 ms. As an example, production of submicron boron carbide powder from gaseous boron trichloride and methane occurs in the strongly endothermic process ... 

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