Compounds of Titanium, Zirconium and Hafnium

Scholz, J., Schlegel, M. and Thiele, K.H. (1987) NMR spectroscopic investigation on the constitution of benzyl and 1-naphthylmethyl compounds of titanium, zirconium and hafnium. Chem. Ber. 120, 1369-1374. 

For an overview of organozirconium and -hafnium chemistry, see P. C. Wailes, R. S. P. Coutts, H. Weigold, Organometallic Chemistry of Titanium, Zirconium, and Hafnium, Academic Press, New York, 1974, p. 302. D. J. Cardin, M. F. Lappeet, C. L. Raston Chemistry of Organozirconium and Hafnium Compounds, John Wiley Sons, New York, 1986, p. 451. 

There has been no recent comprehensive review of this area, although a book on the organometallic chemistry of titanium, zirconium, and hafnium deals, in part, with some of the hydride derivatives (1). In the present review, the first part of the discussion reflects the fact that much of the early work on organotitanium hydrides was, often unknowingly at the time, interwoven with attempts to prepare titanocene, Cp2Ti (Cp = tj3-C5H5). Subsequent sections deal with similar compounds containing an additional metal (e.g., aluminum), miscellaneous titanium hydride compounds, and a summary of the main properties of the above species. 

Thermodynamic data in the area of transition metal chemistry is available, but additional studies would be desirable. One of the early indications that C—Ti bonds are not notoriously weak was obtained from the heats of combustion of Cp2Ti(CH3)2 and Cp2Ti(C6H5)2 with subsequent estimation of the a-bond dissociation energies (250 kJ/mol-1 and 350 kJ/mol 1, respectively)49. From heats of alcoholysis of a number of titanium, zirconium and hafnium compounds, and heats of solution of the products as well as subsidiary data, Lappert estimated heats of formation (AHf°) and thermochemical mean bond energy terms (EM X) of metal—X bondsso> (Table 2). 

One promising approach to the adjustment of coordinative saturation is to alter the number of cyclopentadienyl rings. Since the great bulk of titanium, zirconium, and hafnium organometallic chemistry is that of ( 5115)2X1X2 systems, compounds such as the recently reported ( 5115)2-UCI2 (95) would seem to be ideal precursors. We (96) as well as others (97) have found this to be an incorrect formulation of the reaction product of U U and 2Tl 5H5. When the reaction is carried out in dimethoxy-ethane (DME) we find (96) by NMR that the product is actually a mixture of ( 5H5)3U land ( 5H5)U l3 DME (98). 

The heats of formation A H for ordered stoichiometric alloys were determined with self-consistent linear-augmented calculations [53]. For 50 50% alloys of titanium, zirconium, and hafnium with the heavier 4d and 5d elements the agreement between the theory and experiment was of the order of the scatter of the experimental data. For instance, heat of formation was found for the chemical compound of RuZr to be equal to —0.75 eV/atom according to the calorimetric measurements AH = -0.79eV/atom. 

The cyclopentadienyl derivatives of titanium, zirconium, and hafnium were first described by Wilkinson and coworkers in the early 1950s. These compounds have a tetrahedral geometry, possess a high degree of covalent character and behave like organic acid chlorides in condensation polymerization reactions. Group IVB metallocene-containing materials are known to be active catalytic sites and to offer potentially useful photonic and other properties. ... 

Electrochemical synthesis of titanium, zirconium and hafnium diborides Electrochemical transients Electrochemical reduction of niobium Electrochemical synthesis of new niobium compounds... 

A somewhat surprising group of coordination compounds consists of the volatile heavy metal nitrates, such as those of copper, zinc, mercury, titanium, zirconium and hafnium. The structures of some of these, in the gaseous state, have been determined thus Cu(N03)2 contains two bidentate, almost planar, staggered nitrato groups. Some derivatives of metal nitrates have also been found to be volatile for example, Fe(N03)3 N204 and Al(N03)3 2MeCN.39... 

The Gp. IVA transition elements, titanium, zirconium and hafnium all have the [n — l)d2ws electron configuration. They differ from the transition elements of the later groups in their tendency to form compounds to the exclusion of those in which lower charge numbers occur especially is this true of Zr and Hf. 

The transition metal carbides and nitrides have often been called interstitial compounds [70] however, this is somewhat misleading. The small boron, carbon, or nitrogen atoms certainly occupy octahedral or trigonal prismatic voids of the metal sublattice, but the arrangement of the metal atoms themselves is different from that of the element. In the monocarbides the transition metal atoms show cubic close packing. However, titanium, zirconium, and hafnium are packed hexagonally and vanadium, niobium, and tantalum are body centered cubic [1]. Thus, these monocarbides are inorganic compounds with their individual crystal structures and they should not be considered as an interstitial compound of a transition metal host lattice. 

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... 

Because of the low oxidation state of the metal [M(II)] in the group 4B metallocene dicarbonyl compounds, all of them, perhaps with the exception of (17—C5Me5)2Ti(CO)2 (27), are very air sensitive and decompose rapidly on exposure to air, forming a yellow solid for the titanium compounds and cream-colored solids for the zirconium and hafnium analogs. While the dicarbonyl 27 is indeed air sensitive, its decomposition appears qualitatively to be much slower relative to the other related complexes. 

Recent Group IV chemistry has seen an upsurge in the number of amide derived species, and this has included fluoride derivatives. None of these compounds are of oxidation state -(-III or less, which are the subject of this review, but refer to titanium, zirconium or hafnium where the metal is the +IV state [1,9-12] and, consequently, not covered here. 

All salts of zirconium and hafnium tend to hydrolyze in aqueous solutions, though less so than those of titanium. In highly dilute solutions (<10 " M), Zr and Hf exist as the aqueous ions [M(OH) ] " " +, where n is pH-dependent. The hydration energies are 7001 and 7169kJ moU for Zr and Hf respectively. In chloride, perchlorate, and nitrate solutions, hafnium is less hydrolyzed than zirconium, while the reverse is true in sulfate solutions. This is connected with the lower solubility of hafnium compounds in sulfate solutions, even in only slightly acid media. It should be noted that the sulfate anion has a strong affinity for Zr and Hf... 

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