Zirconium valence

Hafnium [7440-58-6] Hf, is in Group 4 (IVB) of the Periodic Table as are the lighter elements zirconium and titanium. Hafnium is a heavy gray-white metallic element never found free in nature. It is always found associated with the more plentiful zirconium. The two elements are almost identical in chemical behavior. This close similarity in chemical properties is related to the configuration of the valence electrons, and for zirconium and... 

Uranium Purification. Subsequent uranium cycles provide additional separation from residual plutonium and fission products, particularly zirconium— niobium and mthenium (30). This is accompHshed by repeating the extraction/stripping cycle. Decontamination factors greater than 10 at losses of less than 0.1 wt % are routinely attainable. However, mthenium can exist in several valence states simultaneously and can form several nitrosyl—nitrate complexes, some for which are extracted readily by TBP. Under certain conditions, the nitrates of zirconium and niobium form soluble compounds or hydrous coUoids that compHcate the Hquid—Hquid extraction. SiUca-gel adsorption or one of the similar Hquid—soHd techniques may also be used to further purify the product streams. 

Zirconium [7440-67-7] is classified ia subgroup IVB of the periodic table with its sister metallic elements titanium and hafnium. Zirconium forms a very stable oxide. The principal valence state of zirconium is +4, its only stable valence in aqueous solutions. The naturally occurring isotopes are given in Table 1. Zirconium compounds commonly exhibit coordinations of 6, 7, and 8. The aqueous chemistry of zirconium is characterized by the high degree of hydrolysis, the formation of polymeric species, and the multitude of complex ions that can be formed. 

Zirconium forms anhydrous compounds in which its valence may be 1, 2, 3, or 4, but the chemistry of zirconium is characterized by the difficulty of reduction to oxidation states less than four. In aqueous systems, zirconium is always quadrivalent. It has high coordination numbers, and exhibits hydrolysis which is slow to come to equiUbrium, and as a consequence zirconium compounds in aqueous systems are polymerized. 

Zirconium and hafnium have very similar chemical properties, exhibit the same valences, and have similar ionic radii, ie, 0.074 mm for, 0.075 mm for (see Hafniumand hafnium compounds). Because of these similarities, their separation was difficult (37—40). Today, the separation of zirconium and hafnium by multistage counter-current Hquid—Hquid extraction is routine (41) (see Extraction, liquid—liquid). 

Electrolysis. Electrowinning of zirconium has long been considered as an alternative to the KroU process, and at one time zirconium was produced electrolyticaHy in a prototype production cell (70). Electrolysis of an aH-chloride molten-salt system is inefficient because of the stabiUty of lower chlorides in these melts. The presence of fluoride salts in the melt increases the stabiUty of in solution, decreasing the concentration of lower valence zirconium ions, and results in much higher current efficiencies. The chloride—electrolyte systems and electrolysis approaches are reviewed in References 71 and 72. The recovery of zirconium metal by electrolysis of aqueous solutions in not thermodynamically feasible, although efforts in this direction persist. 

Cobalt. Without a doubt cobalt 2-ethyIhexanoate [136-52-7] is the most important and most widely used drying metal soap. Cobalt is primarily an oxidation catalyst and as such acts as a surface or top drier. Cobalt is a transition metal which can exist in two valence states. Although it has a red-violet color, when used at the proper concentration it contributes very Httie color to clear varnishes or white pigmented systems. Used alone, it may have a tendency to cause surface wrinkling therefore, to provide uniform drying, cobalt is generally used in combination with other metals, such as manganese, zirconium, lead, calcium, and combinations of these metals. 

The effect of small valence and large coordination number is further shown by the observation that silicon tetrahedra, which share comers only with aluminum octahedra, share edges with magnesium octahedra (in olivine, chondrodite, humite, clinohumite) and with zirconium polyhedra with coordination number eight (in zircon). 

Cerous iodates and the iodates of the other rare earths form crystalline salts sparingly soluble in water, but readily soluble in cone, nitric acid, and in this respect differ from the ceric, zirconium, and thorium iodates, which are almost insoluble in nitric acid when an excess of a soluble iodate is present. It may also be noted that cerium alone of all the rare earth elements is oxidized to a higher valence by potassium bromate in nitric acid soln. The iodates of the rare earths are precipitated by adding an alkali iodate to the rare earth salts, and the fact that the rare earth iodates are soluble in nitric acid, and the solubility increases as the electro-positive character of the element increases, while thorium iodate is insoluble in nitric acid, allows the method to be used for the separation of these elements. Trihydrated erbium iodate, Er(I03)3.3H20, and trihydrated yttrium iodate, Yt(I03)3.3H20,... 

Titanium is the first member of the 3d transition series and has four valence electrons, 3d24s2. The most stable and most common oxidation state, +4, involves the loss of all these electrons. However, the element may also exist in a range of lower oxidation states, most importantly as Ti(III), (II), (0) and —(I), Zirconium shows a similar range of oxidation states, but the tervalent state is much less stable relative to the quadrivalent state than is the case with titanium. The chemistry of hafnium closely resembles that of zirconium in fact, the two elements are amongst the most difficult to separate in the periodic table. 

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. 

Integrating the data for the aluminum, magnesium, and zirconium orthosilicates leads to the tentative conclusion that the smaller the cation valence, the more severe will be the surface alteration. In general, the calculated ZPC should most closely approximate reality for tri- and quadrivalent cations. It should represent a basic limit for mono- and bivalent cations. 

It was also suggested that a change of valency of the central metal atom from Rh(III) to Rh(I) is an important factor in determining the fragmentation path. The effect of changes in valency has previously been discussed by Shannon and Swan and by Reid et al. for complexes of iron, gold, and zirconium (171, 184). 

From the second cycle the plutonium goes through anion exchange for final purification (Fig. 21.21). The principal problem here is due to ruthenium, which is difficult to remove because of its many valence states. The uranium stream goes through silica sorption primarily to remove zirconium, which seems to be carried along as a colloid. 

Cp2Zr(CH3)(THF)]+ The zirconium oxidation state is 4+ and each Cp ligand donates six electrons. The ligand CTC donates two electrons. The solvent molecule, THF, also donates two electrons, and the total electron count is 12 + 0 + 2 + 2=16. With the covalent model zirconium is in the zero oxidation state and has four electrons Ad2,5s2) in the valence shell. Both Cp and CH3 are considered as radicals and therefore donate five and one electron, respectively. The valence electron count is therefore 4 + 2x5 + 1+ 2-1 = 16. Notice that because of the positive charge, we subtract one electron. 

If it becomes easy now to obtain open structures with di and trivalent transition metals, the litterature is very poor concerning tetravalent cations. However, very recently, two zirconium [72,73] and two titanium [74,75] fluorophosphates were evidenced. The titanium family provides the first example [74], of a mixed valence compound, X iIIITilvF(P04)2, 2 H20 (Fig. 13) in the series of oxyfluorinated solids with an open framework. 

Triangular zirconium mixed-valence complexes stabilized by arenes, [Zr3(/x-Cl)6( ] -C6Me6)3]"+ (n = 1, 2) (see Arene Complexes), have been isolated as salts with the [Al2Cl7] anion (equation 28). Arenes also stabilize divalent hafiiium in the complexes (/x-) -C6H5R)[Hfl2(PMe2Ph)2]2 (R = H, Me), which have a piano stool structure (see Piano Stool Structure) ... 

Refractory metals, such as tantalum and zirconium, can be deposited from their fluorides in a molten salt bath. In the case of zirconium, for example, the bath consists of ZrF or ZrF in a KF/NaF/LiF mixture. The alkali fluorides are employed to increase conductivity and decrease the melting point. Even so, these baths are operated at about 800 C. Good deposits have been reported as long as the right valency was chosen for each metal (3 for Mo and V, 4 for Nb and Zr, 5 for Ta). The bath must be operated in a pure argon atmosphere, and impurities must be strictly excluded. It siiould be obvious that the operation of such baths is expensive and control is difficult. Thus their use is limited either for research purposes or for highly specialized applications, where cost is of secondary importance. 

---