Zirconium and hafnium compounds

The principal valence of zirconium and hafnium is -1-4. Halides of valence +2 and -(-3 have been prepared, but they are of less practical importance as they disproportionate when heated and react with water in aqueous solution. 

TaUe 7.4 Compositioii specifications for ziiconium sponge and zirconium alloys 

Form Common name Sponge Zirconium sponge Ingots  

ASTM grade R60001 R60802 R60804 R60901 

ASTM specification B349-73 B350-73 B350-73 B350-73 

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D. J. Cardin, M. F. Lappert, and C. L. Raston, Chemistry of Organo-Zirconium and -Hafnium Compounds, Hasted Press, Division of John Wiley Sons,... 

The crystal structures of Hf 2 (OH) 2 (S0O 3 (H2O) i, (14) and Ce2(0H)2(S0i,)3 (H20)it (14) also have been determined and found to be isomorphous to the zirconium compound. The cell constants for this series of four isomorphous compounds reflect the effect of the ionic radii on the dimensions of the unit cell. The values for these cell constants are in Table II. Thus, the cell constants for the zirconium and hafnium compounds are nearly identical and smaller than the cell constants for the cerium and plutonium compounds which are also nearly identical. This trend is exactly that followed by the ionic radii of these elements. 

The refractory carbides NbC and TaC and the nitride NbN, each with the sodium chloride structure, are also similar to the corresponding zirconium and hafnium compounds. Finally, it may be noted that the same group of complexing agents form complexes with Nb(V) and Ta(V) as with Zr(IV) and Hf(IV) (for example, fluoride, hydrogen peroxide, acetylacetone, and 1,2-dihydroxybenzene). 

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). 

As exemplified by the reactions of Schemes 1 and 4, fluorotitanium compounds could open new possibilities for metal-catalyzed processes. Their fascinating structural diversity [7] as well as further catalytic possibilities in the field of olefin polymerizations [7i, 16] have been put forward by the pioneering work of Roesky, Noltemeyer and co-workers. Similar properties were also exhibited by the analogous zirconium and hafnium compounds [7b,i]. A Zr binaphtholate has already been successfully applied for the enantioselective allylstannylation of aldehydes [2f], Buch-wald and co-workers successfully used a chiral titanocene difluoride as precursor for the corresponding Ti(lII) hydride, a very efficient catalyst for the enantioselective hydrosilylation of imines [17]. 

The uses of zirconium and hafnium compounds based on their thermal and corrosion stability are described in related sections (see below). The most dramatic use of zirconium... 

Structures of di- and monovalent zirconium and hafnium compounds contain octahedra, chains, or layers of metal atoms with metal-metal bonds. It is this that differentiates these compounds from those of M (MX3 or molecular M2X6L4 complexes) with isolated M-M bonds. A recent reinvestigation of the ZrL structure showed two distinct Zr-Zr distances (3.507 and 3.172 A) the latter is typical for a Zr-Zr bond (see Bond Length). 

The thorium compound (10 g.) reacts with water (100 ml.) to give an insoluble portion for which the oxalate/thor-ium mol ratio is 2.67 and a soluble portion which can be recovered by precipitation with alcohol and which has an oxalate/thorium mol ratio of 2.99. The remainder of the oxalate is lost by replacement by hydroxide ion and water and stays in solution. The zirconium and hafnium compounds, on the other hand, can be recrystallized from water with very little loss of oxalate. 

D. J. Cardin, M. F. Lappert and C. L. Raston, Chemistry of Organo-zirconium and -hafnium Compounds , Wiley, New York, 1986 R. S. P. Courts, P. C. Wailes and H. Weigold, Organometallic Chemistry of Titanium, Zirconium, Hafnium , Academic Press, New York, 1974 G. E. Coates, M. L. H. Green and K. Wade, in Organometallic Compounds , Chapman and Hall, London, 1968, vol. 2. 

Zero-valent zirconium and hafnium compounds remain relatively rare, owing to the strong thermodynamic driving force for the second and third row metals to attain a higher oxidation state. Despite this obstacle, examples of formally zero-valent compounds have been reported and characterized. The majority of these are arene complexes, whose syntheses and resulting chemistry have been reviewed.1,2 In addition to arene compounds, formally zero-valent butadiene complexes have also been described and are the subject of a rather comprehensive review.3 The focus of this section will be on compounds that have not been covered. 

Allenes have also been used to stabilize low-valent zirconium and hafnium compounds. This chemistry has been the subject of a recent review.74... 

Introduction.—A text describing the co-ordination compounds of zirconium and hafnium with organic ligands has been published,and the history of the discovery of hafnium and its separation from zirconium has been described. There have been reviews of the structures of zirconium and hafnium compounds, of hydrozircona-tion, and of the organometallic chemistry of zirconium and hafnium. ... 

Zirconium has a high corrosion resistance and low cross-section for neutron capture (see Section 2.4) and is used for cladding fuel rods in water-cooled nuclear reactors. For this application, Zr must be free of Hf, which is a very good neutron absorber. The main use of pure Hf is in nuclear reactor control rods. Zirconium and hafnium compounds possess similar lattice energies and solubilities, and their complexes have similar stabilities this means that... 

Organotransition Metal Chemistry. Fundamental Concepts and Applications, Ed. A.Yamamoto, Wiley Interscience New York. 1986. Chemistry of Organo-zirconium and -hafnium Compounds, D.J. 

Zr(BH4)4 at 25 °C indicate the equivalence of all of the hydrogen atoms owing to rapid intramolecular exchange between the bridging and terminal positions. and B spectra of the zirconium and hafnium compounds are closely similar. The spectra consist of a quartet of equally intense lines due to spin-spin splitting by boron-11 (1 = 1 — H) = 90Hz), with... 

Cardin, D. J. Lappert, M. F. Raston, C. L. Chemistry of Orcrano-Zirconium and -Hafnium Compounds. Ellis Horwood Limited, West Sussex, England, 1986, Chapter 10. 

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