Zirconium divalent

Distorted variants, similar to the distorted variants of the NiAs type, are known for the Cdl2 type. For example, Zrl2 has a distorted Cdl2 structure in which the Zr atoms form zigzag chains. Therefore, every Zr atom is involved in two Zr-Zr bonds which is in accordance with the d2 configuration of divalent zirconium. 

Zirconium exhibits quadrivalency in most of its compounds although divalent and trivalent compounds also exist. Zirconium reacts with oxygen to form zirconium oxide, Zr02. In powder form, Zr metal ignites spontaneously forming oxide. Solid metal, however, is stable in air at ordinary temperatures, but reacts slowly at 200°C. Reaction is rapid at high temperatures. 

For several ions plausible structures require trivalent or divalent zirconium, e.g., [C5H5ZrCl]+, [(C5H5)2Zr]+, [ZrCl2]+, and [(C5H5)(C1) ZrOZrCl]+. In the last ion the resonance structure includes contributions in which both zirconium atoms are formally trivalent and others in which one is tetravalent and the other divalent. 

In this chapter we will review the synthesis, structural aspects, and basic chemical properties of formally divalent and trivalent titanium and zirconium metallocene complexes. We have restricted our coverage to the low-valent bis(rj-cyclopentadienyl) and related metallocenes metal halide complexes and organometallic mixed metal systems will not be discussed here. We have not attempted to present an exhaustive coverage of the field. Rather, our aim has been to describe critically and to evaluate the often confusing chemistry that has been reported for the reactive low-valent titanium and zirconium metallocenes. More general reviews (7) and a book (2) on the organometallic chemistry of titanium, zirconium, and hafnium have been published. 

Except for BeF2, glasses based on divalent fluorides show a broader IR transparency than glasses based on indium or zirconium, as reported for ZnSB and ZnSBC in Fig. 1 [10]. They exhibit a full transparency up to 9 /an for the former and 10 nm for the latter which is a pure MF2 fluoride glass containing Zn, Sr, Ba, and Cd. 

Aluminum oxide is the only trivalent oxide that has been used to form a ceramic some heat treatment is needed. Kingery claims to have observed a setting reaction between trivalent iron oxide and phosphoric acid, but this reaction may have been caused by traces of magnetite in the trivalent oxide. Pure trivalent iron oxide such as hematite (Fe203) does not react with phosphoric acid. Overall, trivalent metal oxides have a solubility that is only marginal and falls below that of even sparsely soluble divalent oxides, while the solubility of oxides of most quadrivalent metals (zirconium is an exception) is too low to form a ceramic. 

Triangular zirconium mixed-valence complexes stabilized by arenes, [Zr3(/x-Cl)6( ] -C6Me6)3]"+ (n = 1, 2) (see Arene Complexes), have been isolated as salts with the [Al2Cl7] anion (equation 28). Arenes also stabilize divalent hafiiium in the complexes (/x-) -C6H5R)[Hfl2(PMe2Ph)2]2 (R = H, Me), which have a piano stool structure (see Piano Stool Structure) ... 

The crystalline form of interest in Zr-based ceramic compounds is the cubic fluorite structure based on the mineral CaF2. In this structure, consisting of interpenetrating face-centered-cubic and simple cublic arrays of cations (Zr ) and anions (O ), respectively, oxygen ion conductivity is enhanced by replacing zirconium (Zr ) ions on the cation lattice with soluble dopant cations having a valence less than 4, typically divalent (Mg, Ca ) and trivalent (Y, Yb , Sc ) cations. These dopants, which are in solid solution, are incorporated into the zirconia structure by the following types of defect reaction ... 

Treatment of zirconocene dichloride, 50, with 2 equiv. of an appropriate alkyllithium or Grignard reagent generates transient zirconocene olefin complexes that upon loss of alkene provide access to zirconocene, 109, and the powerful reduction chemistry of divalent zirconium.48 Owing to the utility of this reagent in organic synthesis and organometallic reactions, the low-temperature alkylation of zirconocene dichloride, 50, with BunLi has been... 

As was seen in the previous section, addition of excess olefin to formally divalent zirconium or hafnium alkene complexes usually induces coupling to form the corresponding metallacyclopentane. Interestingly, metallacycle... 

The sterically demanding alkyne, Me3SiC=CSiMe3, has been widely used to stabilize a range of zirconocene complexes while also serving as a convenient leaving group to access the rich chemistry of divalent zirconium. These results have been the subject of a comprehensive review. Representative reactivity will be presented here. 

Reactivity studies reveal that the bis(indenyl)zirconium sandwich complexes serve as isolable, modular precursors to the rich chemistry of divalent zirconocene. Addition of 1 atm of carbon monoxide generates the bis(indenyl)-zirconocene dicarbonyl complex, 307, while treatment with N,N-dimethylaminopyridine results in C-H activation to form the zirconocene pyridyl hydride complex 308. Crystallographic characterization of both complexes as well as multinuclear NMR spectroscopy establishes that the more familiar 77s, 77s hapticity for the indenyl ligands has been restored. Alkyne coupling reactions are also observed as addition of 2-butyne to 302 allows observation of the alkyne complex, 208, and ultimately the zirconacyclopentadiene 309 (Scheme 50).104... 

The product is derived by oxidation of a 2-naphthyl C—H bond to a divalent zirconocene species. The p-naphthyl group is 2-r) to one zirconium atom and 1,2-r/ to the other. 

From the study of cell potentials, the equilibrium constants for various reductions in the equimolar sodium-potassium chloride melt were calculated 550) (Table XXVII). The high values of the equilibrium constants indicate that zirconium metal should reduce any tetravalent or trivalent zirconium present to the divalent species. The reduction in both cases should he quantitative. It can also be predicted that zirconium trichloride in solution does not disproportionate to any significant extent, and that the reaction between zirconium dichloride and tetrachloride to produce zirconium trichloride proceeds almost to completion. 

This is a study of the solubility of an uncharacterised zirconium solid, presumably Zr(OH)4(am), using the tyndallometric technique. The study was carried out at 20°C and the precipitation experiments were conducted in 50% seawater (salinity = 18.29%o). Results from the study indicated that at low pH (< 3.5), the solution species was divalent and, above pH 4, the solubility was invariant indicating the presence of the neutral species, Zr(OH)4. In this region, the solubility was found to be 2.5 x 10 mol-L from this the authors estimated log, to be equal to -4.6. 

One category of dense proton conducting membranes that has received considerable attention in the preceding decade is proton conducting perovskite type oxide ceramics [4-6]. The stoichiometric chemical composition of perovskites is represented as ABO3, where A is a divalent ion (A +) such as calcium, magnesium, barium or strontium and B is a tetravalent ion (B +) such as cerium or zirconium. Although simple perovskites such as barium cerate (BaCeOs) and strontium cerate... 

Zirconium forms only one important oxide, zirconia Zr02, which is amphoteric in character. The normal zirconium salts, like ZrC, are readily hydrolysed in solution giving rise chiefly to zirconyl salts, containing the divalent radical ZrO. The zirconates, e.g. Na2Zr03, are best produced from Zr02 by fusion methods. Zirconium also readily forms complex ions like hexafluorozirconate(IV) [ZrF ] , produced by heating zirconia with potassium hydrogen fluoride. 

An altered zircon, cyrtolite, which is a hydrated zirconium silicate in which part of the zirconium is replaced by hafnium, and divalent and trivalent metals, is used as the raw material. This ore, although not very abundant, is used as the source material rather than the zircon or baddeleyite (1) because of its higher hafnium content (5 to 9 per cent compared to 2 per cent or less for the normal ores), and (2) because it is easily susceptible to acid attack. Ordinarily silicates are not easily attacked by acid treatment and must be handled by some fusion method. However, Urbain reported successful extraction by sulfuric acid at 65° of malacon, an altered zircon, and other investigators have noted that cyrtolite also yields to sulfuric acid treatment. 

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