Hafnium trivalent

Whereas zirconium was discovered in 1789 and titanium in 1790, it was not until 1923 that hafnium was positively identified. The Bohr atomic theory was the basis for postulating that element 72 should be tetravalent rather than a trivalent member of the rare-earth series. Moseley s technique of identification was used by means of the x-ray spectra of several 2ircon concentrates and lines at the positions and with the relative intensities postulated by Bohr were found (1). Hafnium was named after Hafma, the Latin name for Copenhagen where the discovery was made. 

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. 

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. 

The number of bonding electrons at the beginning of the 5d series may be inferred from comparing the measured atomic volumes of rare earths and transition metals. On the right in fig. 1, Yb is clearly a divalent lanthanide and, therefore, has two valence electrons. Similarly, lutetium is a trivalent lanthanide and has three valence electrons. On the left in the same figure, these two elements are seen to be also the first two members of the 5d transition series and the third, hafnium, will have four valence electrons. 

The cohesive energies (Brewer 1975) of the (non-f) elements in the left part of the periodic table show representative trends, being consistently close to 40 kcal/mol for divalent metals (such as barium and strontium), about 100 kcal/mol for trivalent metals (such as lanthanum and yttrium) and about 145 kcal/mol for tetravalent metals (such as hafnium and zirconium). 

By contrast the introduction of trivalent cations, such as indium, into lithium hafnium phosphate gives only a modest increase in the conductivity of the sample. The room temperature conductivity of Lii 2Hfi.gIno.2(P04)3 is 10 S cm and this material is only marginally more conducting that the indium-free composition LiHf2(P04)3. Both materials shows a similar temperature-independent activation energy for conduction of ca 0.44 This modest enhancement in room tem-... 

---