Hafnium tetravalent

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. 

The only crystalline phase which has been isolated has the formula Pu2(OH)2(SO )3(HaO). The appearance of this phase is quite remarkable because under similar conditions the other actinides which have been examined form phases of different composition (M(OH)2SOit, M=Th,U,Np). Thus, plutonium apparently lies at that point in the actinide series where the actinide contraction influences the chemistry such that elements in identical oxidation states will behave differently. The chemistry of plutonium in this system resembles that of zirconium and hafnium more than that of the lighter tetravalent actinides. Structural studies do reveal a common feature among the various hydroxysulfate compounds, however, i.e., the existence of double hydroxide bridges between metal atoms. This structural feature persists from zirconium through plutonium for compounds of stoichiometry M(OH)2SOit to M2 (OH) 2 (S0O 3 (H20) i,. Spectroscopic studies show similarities between Pu2 (OH) 2 (SOO 3 (H20) i, and the Pu(IV) polymer and suggest that common structural features may be present. 

The chemical properties of hafnium are very much similar to those of zirconium. In aqueous solutions, the metal exists in tetravalent state. The elec-... 

The general area of zirconacyclopentane chemistry has been reviewed.76 Because these molecules are best viewed as zirconium or hafnium dialkyl complexes, and hence as tetravalent metal centers, this renders them beyond the scope of this review. It should be noted that these molecules display a rich reaction chemistry, serving as initiators for olefin polymerization,77-79 reagents for organic methodology,57,80-86 and key intermediates in natural product synthesis.87,88... 

Zirconium (Zr, CAS 7440-67-7, atomic number 40, atomic mass 91.22) has a melting point of 1852 °C and a boiling point of 4377 °C. It is a hard, lustrous, silvery metal, in contrast to fine zirconium powder, which is black. Zirconium belongs to Subgroup IV of the Periodic Table of the elements, between the elements titanium and hafnium - two metals with which it is often found in nature. Zirconium has oxidation states ranging from II to IV, of which the tetravalent is relatively stable and abundant (Venugopal and Luckey 1979). Zirconium is very corrosion-resistant and is unaffected by alkalis or acids (except for HF). 

Since none of the lanthanides forms a tetravalent metal, it is somewhat more difficult to obtain accurate values for A , iv However, from fig. 1 it is quite clear that cerium as a tetravalent metal should have a cohesive energy close to 145 kcal/ mol relative to its tetravalent atomic state d s. For the tetravalent element after lutetium in the Periodic Table, i.e. hafnium, the experimental cohesive energy is 148.4 kcal/mol (Brewer 1975). Therefore all the lanthanides should in their hypothetical tetravalent metallic state have a cohesive energy of about 145-148 kcal/mol relative to the proper tetravalent atomic state. This means that if the atomic excitations f" ds d s were known for the lanthanides we could easily... 

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). 

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