Reduced zirconium halides

Of course, the chemistry of zirconium cluster phases has been well described and reviewed in the literature [1-4]. Apart from a very few examples, mostly in the binary halides, almost all reduced zirconium halides contain octahedra of zirconium atoms centred on an interstitial atom Z. Several possible and experimentally realized Z include H, Be-N, K, Al-P, and the transition metals Mn-Ni. All these compounds have the general formula Ax"[(Zr6Z)Xi2X[J], with a " = alkali or alkaline earth metal cation, X=C1 Br or I, X =inner edge-bridging halide [5], X =outer exo-bonded halide, and 0[Pg.61]

Fig. 5.1 [(Zr6Z)XTg] cluster unit in reduced zirconium halides.

X. Xie and T. Hughbanks (2000) Inorganic Chemistry, vol. 39, p. 555 - Reduced zirconium halide clusters in aqueous solution . 

In this table, the free energy of formation, AGf of the chloride of these metals is listed for four different temperatures. As can be seen, the values are more negative than that of hydrogen chloride. These metals can be used to reduce the halides of titanium, zirconium, or hafnium, whereas hydrogen, as mentioned above, cannot do so readily. In order to be useful in CVD, the by-product chloride must be volatile at the deposition temperature. This may rule out the use of sodium or potassium, which evaporate above 1400°C. 

Fig. 6.1b) in which twelve inner ligands bridge the edges of the Me octahedron, and six outer ligands occupy apical positions, predominate. These units are found in reduced zirconium, niobium, tantalum, and rare-earth halides, and niobium, tantalum, molybdenum and tungsten oxides [la, 6, 10]. 

Only a small number of zirconium(III) and hafnium(III) complexes are known. Nearly all of these are metal trihalide adducts with simple Lewis bases, and few are well characterized. Just one zirconium(III) complex has been characterized structurally by X-ray diffraction, the chlorine-bridged dimer [ ZrCl PBu,) ]- Although a number of reduced halides and organometallic compounds are known in which zirconium or hafnium exhibits an oxidation state less than III, coordination compounds of these metals in the II, I or 0 oxidation states are unknown, except for a few rather poorly characterized Zr° and Hf° compounds, viz. [M(bipy)3], [M(phen)3] and M Zr(CN)5 (M = Zr or Hf M = K or Rb). 

Calcium metal is an excellent reducing agent for production of the less common metals because of the large free energy of formation of its oxides and halides. The following metals have been prepared by the reduction of their oxides or fluorides with calcium hafnium (22), plutonium (23), scandium (24), thorium (25), tungsten (26), uranium (27,28), vanadium (29), yttrium (30), zirconium (22,31), and most of the rare-earth metals (32). 

Many highly reduced halides of scandium, yttrium, and zirconium have been found to have infinite metal-metal bonded chains.169 Zirconium chloride, for example, contains double metal layers alternating with double chlorine layers (Fig. 16.68). It was dis-... 

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. 

The reactions of metals to form stable halides are important for various reasons. The metal halides generally have low boiling points and high volatiUty. For this reason, they are used in several important processes for the production and refining of metals, such as the reactive metals titanium and zirconium. These metals are produced using the Kroll process, in which the metal oxide is converted to metal chloride or fluoride, which is then reduced to metal. This route avoids several formidable difficulties involved in the reduction of the oxides of these metals. Details of these processes can be found in extractive-metallurgy textbooks. 

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