Group 4 zirconium and hafnium

In a finely divided form, Hf and Zr metals are pyrophoric, but the bulk metals are passivated the high corrosion resistance of Zr is due to the formation of a dense layer of inert Zr02- The metals are not attacked by dilute acids (except HF) unless heated, and aqueous alkahs have no effect even when hot. At elevated temperatures, Hf and Zr combine with most non-metals (e.g. equation 22.11). 

More is known about the chemistry of Zr than Hf, the former being more readily available (see Section 22.2). 

Much of the chemistry concerns Zr(TV) and Hf(IV), the lower oxidation states being less stable with respect to oxidation than the first group member, Ti(III). In aqueous solutions, only M(IV) is stable although not as M, even though tables of data may quote half-equation 22.12 solution species (see below) depend upon conditions. 

Stabilization of low oxidation states of Zr and Hf by tt-acceptor ligands is discussed in Chapter 23. 

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Titanium is an element of group 4 of the periodic table. It is in the same group as zirconium and hafnium. It has a high similarity to silicon which was the same group in the old periodic table. Titanium exists in 5600 ppm in the Earth s crust [1], it is the fourth largest element after iron, aluminium and magnesium as common use metal. The titanium deposits are approx. 340 million tons or more [2], The span of life as a metal resource is three thousand years or more, the ranking of the life of resources as practical metals is the second after iron. 

The chemistry of hafnium has not received the same attention as that of titanium or zirconium, but it is clear that its behaviour follows that of zirconium very closely indeed with only minor differences in such properties as solubility and volatility being apparent in most of their compounds. The most important oxidation state in the chemistry of these elements is the group oxidation state of +4. This is too high to be ionic, but zirconium and hafnium, being larger, have oxides which are more basic than that of titanium and give rise to a more extensive and less-hydrolysed aqueous chemistry. In this oxidation state, particularly in the case of the dioxide and tetrachloride, titanium shows many similarities with tin which is of much the same size. A large... 

The effect of the metals used was then examined (Table 5.4). When the group 4 metals, titanium, zirconium, and hafnium, were screened it was found that a chiral hafnium catalyst gave high yields and enantioselectivity in the model reaction of aldimine lb with 7a, while lower yields and enantiomeric excesses were obtained using a chiral titanium catalyst [17]. 

We first studied group 4 metals (titanium, zirconium and hafnium) supported on a silica dehydroxylated especially at 700 °C (Table 3.8). Following the laboratory-developed strategy, surface-species have been well-characterized by classical techniques (IR, solid-state NMR gas evolvement, reactivity, etc.). Catalysis results show that titanium is the most active even if its activity is far less than that of homogeneous catalysts. In addition, an important amount of metal was lost by lixiviation even if this phenomenon seemed to stop after a certain time. 

Group 4 In ordet of increasing atomic number, ihese are titanium, zirconium, and hafnium. The elements of this group are characterized by the presence ol two electrons in an outer shell Although titanium and zirconium also have other valences, all of ihe dements in this group have u 4+ valence in eunimnn. 

The synthesis of the transactinides is noteworthy from a chemical and a nuclear viewpoint. From the chemical point of view, rutherfordium (Z = 104) is important as an example of the first transactinide element. From Figure 15.1, we would expect rutherfordium to behave as a Group 4 (IVB) element, such as hafnium or zirconium, but not like the heavy actinides. Its solution chemistry, as deduced from chromatography experiments, is different from that of the actinides and resembles that of zirconium and hafnium. More recently, detailed gas chromatography has shown important deviations from expected periodic table trends and relativistic quantum chemical calculations. 

Addition of thiocyanate ions to chloride or perchlorate solntions of zirconium and hafnium yields complexes containing from one to eight isothiocyanate groups per metal atom. These systems are of interest because of the importance of thiocyanate complexes in the extraction and separation of the elements. IR spectroscopy indicates that M-N bonds are present in the violet (Zr) and pink (Hf) complexes [NEt4]2[M(NCS)6] analogous complexes have been obtained with alkali metal cations. In the presence of pyridine, the dodecahedral Zr(bipy)2(NCS)4 complex is produced see Ammonia N-donor Ligands). 

The organometallic chemistry of titanium is dominated by complexes in the +IV oxidation state and in comparison there are relatively few examples of titanium complexes in the +III oxidation state. For information on organotitanium(iv) see Chapter 4.05. However, examples of titanium(lll) complexes are more common than examples of titanium complexes in lower oxidation states (for information on organotitanium in oxidation states 0 to II see Chapter 4.03) and titanium(m) chemistry is considerably more advanced than the chemistry of the heavier group 4 metals, zirconium and hafnium in the +m oxidation state. For information on organozirconium(m) and organohafnium(m) see Chapter 4.07. 

Compound (CsH4SiMe2NBut)TiCl2 has been synthesized and used as pre-catalyst for ethylene polymerization. The activities and the properties of the polymers have been compared to similar zirconium and hafnium derivatives.720 The consequences of anion-cation interactions on the activity of GGG group 4 metal complexes in olefin polymerizations have been explored for a series of zirconocene derivatives as well as the cationic species [(C5Me4SiMe2NBut)TiMe]+ with the sterically congested tris(perfluorobiphenyl)fluoroaluminate as the counteranion.721 The co-polymerization of ethylene and 1-butene by (CsMe iMe Bu TiC in the presence of... 

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