Zirconium behavior

Hafnium [7440-58-6] Hf, is in Group 4 (IVB) of the Periodic Table as are the lighter elements zirconium and titanium. Hafnium is a heavy gray-white metallic element never found free in nature. It is always found associated with the more plentiful zirconium. The two elements are almost identical in chemical behavior. This close similarity in chemical properties is related to the configuration of the valence electrons, and for zirconium and... 

Because of its low neutron absorption, zirconium is an attractive stmctural material and fuel cladding for nuclear power reactors, but it has low strength and highly variable corrosion behavior. However, ZircaHoy-2, with a nominal composition of 1.5 wt % tin, 0.12 wt % iron, 0.05 wt % nickel, 0.10 wt % chromium, and the remainder zirconium, can be used ia all nuclear power reactors that employ pressurized water as coolant and moderator (see... 

Above 40 wt % hydrogen content at room temperature, zirconium hydride is brittle, ie, has no tensile ductiHty, and it becomes more friable with increasing hydrogen content. This behavior and the reversibiHty of the hydride reaction are utilized ki preparing zirconium alloy powders for powder metallurgy purposes by the hydride—dehydride process. The mechanical and physical properties of zirconium hydride, and thek variation with hydrogen content of the hydride, are reviewed in Reference 127. 

As a hard, high melting carbide and possible constituent of UC-fueled reactors, zirconium carbide has been studied extensively. The preparation, behavior, and properties of zirconium and other carbides are reviewed in Reference 132, temperature-correlated engineering property data in Reference 133 (see also Carbides). 

Zirconium tetrabromide [13777-25-8] ZrBr, is prepared direcdy from the elements or by the reaction of bromine on a mixture of zirconium oxide and carbon. It may also be made by halogen exchange between the tetrachloride and aluminum bromide. The physical properties are given in Table 7. The chemical behavior is similar to that of the tetrachloride. 

Zirconium tetraiodide is the least thermally stable zirconium tetrahaUde. At 1400°C, it disproportionates to Zr metal and iodine vapor. This behavior is utilized in the van Arkel-de Boer process to refine zirconium. As with the tetrachloride and tetrabromide, the tetraiodide forms additional adducts with gaseous ammonia which, upon heating, decompose through several steps ending with zirconium nitride. 

Monovalent Halides. Zirconium monochloride [14989-34-5], ZrCl, was discovered during electrorefining studies of zirconium in a SrCl2—NaCl—ZrCl4 melt intended to produce pure ductile hafnium-depleted zirconium from cmde zirconium anodes (180—181). The monochloride is also called Zirklor. It is obtained as black flakes with a graphite sHp-plane behavior and was proposed as a lubricant (182,183). 

Hydrous Oxides and Hydroxides. Hydroxide addition to aqueous zirconium solutions precipitates a white gel formerly called a hydroxide, but now commonly considered hydrous zirconium oxide hydrate [12164-98-6], 7 0 - 112 0. However, the behavior of this material changes with time and temperature. 

The properties and behavior of double phosphates such as — Y)3 (216), sodium—zirconium phosphate—siUcates (217), and... 

SO3 H20 and Ce(S0i,)2 remain. This behavior is similar to that of zirconium and the same conclusions with respect to plutonium can be drawn. 

Similar are the behaviors of aluminum trichloride, zirconium chloride and many other chlorides. There are, however, chlorides like sodium chloride, which do not undergo hydrolysis readily. Only at 600 to 900 °C does the reaction... 

Although the bonding mode of butadiene to zirconium is -it2, -p4, as revealed by X-ray analysis, in a number of reactions and under certain conditions it behaves like a a2, it, p4 butadiene, thus the zirconium-butadiene functionality behaves as though containing two Zr-C a bonds. This behavior is illustrated in the reactions listed in Scheme 9. 

A special case of the chain back skip polymerization mechanism and therefore an entirely different polymerization behavior was observed for differently substituted asymmetric complexes (for example catalyst 3). Although asymmetric in structure, these catalysts follow the trend observed for C2-symmetric metallocenes [20], Chien et al. [23] reported a similar behavior for rac-[l-(9-r 5-fluorenyl)-2-(2,4,7-trimethyl-l-ri5-indenyl)ethane]zirconium dichloride and attributed this difference in the stereoerror formation to the fact that both sides of the catalyst are stereoselective thus isotactic polypropylene is obtained in the same manner as in the case of C2-symmetric metallocene catalysts. 

As with the above pyrrolidine, proline-type chiral auxiliaries also show different behaviors toward zirconium or lithium enolate mediated aldol reactions. Evans found that lithium enolates derived from prolinol amides exhibit excellent diastereofacial selectivities in alkylation reactions (see Section 2.2.32), while the lithium enolates of proline amides are unsuccessful in aldol condensations. Effective chiral reagents were zirconium enolates, which can be obtained from the corresponding lithium enolates via metal exchange with Cp2ZrCl2. For example, excellent levels of asymmetric induction in the aldol process with synj anti selectivity of 96-98% and diastereofacial selectivity of 50-200 116a can be achieved in the Zr-enolate-mediated aldol reaction (see Scheme 3-10). 

The interesting feature of 20 is that the Si-H interaction occurs for the set of electron-donating ancillary ligands Cp2/PMe3. Thus, the only factor that can, in principle, account for the different behavior of titanium and its heavier analogs in these reactions is the contracted nature of the titanium d-orbitals and hence the less effective backdonation from metal as discussed in Section II.B. The nonclassical nature of the zirconium complex 19 compared with neutral 18 can be then attributed to the presence of a positive charge. 

Having set out the properties of tantalum and zirconium hydride toward C-H bond activation of alkanes we now describe the catalytic hydrogenolysis of C-C bonds. It was previously shown in the laboratory that supported-hydrides of group 4 metals, and particularly of zirconium, catalyze the hydrogenolysis of alkanes [21] and even polyethylene [5] into an ultimate composition of methane and ethane. However, to our initial surprise, these zirconium hydrides did not cleave ethane. (=SiO)2Ta-H also catalyzes the hydrogenolysis of acyclic alkanes such as propane, butane, isobutane and neopentane. But, unlike the group 4 metals, it can also cleave ethane [10], Figure 3.7 illustrates this difference of behavior between (=SiO)2Ta(H) and [(=SiO)(4.j,)Zr(H) ], x= or 2). With Ta, propane is completely transformed into methane by successive reactions, while with Zr only equimolar amounts of methane and ethane are obtained. 

Only one example of electrophilic behavior of silicon-stabilized lithiooxiranes is reported. Intermolecular C—Li insertion followed by Li20 elimination occurs by raising the temperature, and ( ) vinylsilanes are obtained stereoselectively (Scheme 80). Reaction of lithiooxiranes with aluminum , zirconium and silicon reagents leads to the corresponding ate complexes, which undergo 1,2-metallate rearrangements. 

Other metals give initial distributions of products equivalent to a combination of the distributions described in 1 and 2 i.e., substantial amounts of both C2H5D and C2D6 are formed, and the distribution plotted against deuterium content is U-shaped. The values of M for the metals showing this behavior are zirconium 2.3, chromium 2.5, vanadium 2.6, and platinum 3.5. 

These examples all involve group IVa metal complexes, suggesting that the desired behavior might be possibly an attribute of early transition metal hydride complexes, an area that has been studied considerably less than complexes of metals that are further to the right of the early transition metals on the periodic table. Indeed, as discussed in this volume (see Chapter 10), a zirconium hydride complex recently has been found to reduce CO to methanol among other products. 

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