Zirconium complexes aqueous solution

This chapter reviews the literature up to approximately January 1984. Previous comprehensive reviews of the chemistry of zirconium and hafnium are available, and annual surveys of the inorganic and coordination chemistry of these elements have been published since 1981 The present treatment emphasizes the chemistry of discrete, isolable complexes, although some attention has been given to the extremely complex aqueous solution chemistry of Zr and Hf. In general, zirconium forms slightly more stable complexes in solution than does hafnium stability constants may be found in standard compilations.In cases where the structural chemistry of discrete complexes is closely related to that of chain, layer or extended three-dimensional structures, the discussion of the discrete complexes has been set in the broader context the section on fluorometallate complexes is a case in point. Organometallic compounds of zirconium and hafnium have been excluded almost entirely. 

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. 

The extraction of metals by liquid amines has been widely investigated and depends on the formation of anionic complexes of the metals in aqueous solution. Such applications are illustrated by the use of Amberlite LA.l for extraction of zirconium and hafnium from hydrochloric acid solutions, and the use of liquid amines for extraction of uranium from sulphuric acid solutions.42,43... 

Zirpro A process for flame-proofing textiles by treating them with aqueous solutions of zirconium complexes. Wool is treated with aqueous potassium hexafluorozirconate and citric acid. Developed by the International Wool Secretariat, Yorkshire, now based in Melbourne, Australia. 

No zirconium(III) complexes with oxygen donor ligands have been isolated. However, the electronic absorption spectra of aqueous solutions of Zrl3 have been interpreted in terms of the formation of aqua complexes (equation 4).29 The spectrum of a freshly prepared solution of Zrl3 exhibits a band at 24 400 cm-1, which decays over a period of 40 minutes, and a shoulder at 22000 cm-1, which decays more rapidly. The 24400 cm-1 band has been assigned to [Zr(H20)6]3+, and the 22000 cm-1 shoulder has been attributed to an unstable intermediate iodo-aqua complex. If it is assumed that the absorption band of [Zr(H20)6]3+ is due to the 2T 2Ee ligand-field transition, the value of A is 24 400 cm. This corresponds to a A value of 20 300 cm-1 for [Ti(H20)6]3+ 30 and 17 400 cm-1 for the octahedral ZrCl6 chromophore in zirconium(III) chloride.25... 

In aqueous solution, zirconium(IV) and hafnium(IV) form complexes M(NCS)4-", where n = 1-8.104 Selective extraction of hafnium thiocyanate complexes from acidic aqueous solution by methyl isobutyl ketone is a widely used industrial method for the separation of zirconium and hafnium. Separation methods have been reviewed by Vinarov.105... 

Because K2HfF6 is —1.7 times more soluble in water than K2ZrF6,528 zirconium and hafnium can be separated by fractional crystallization of the hexafluorometallates. This approach is used on an industrial scale in the USSR.286 Conductance measurements on aqueous solutions of M2MF6 (M = K, Rb, or Cs) indicate very little hydrolysis of the [MF6]2- ions.529 Alkaline hydrolysis of potassium and ammonium fluorozirconates yields crystalline M ZrF3(0H)2-H20 complexes, which are easily dehydrated to M ZrF3(OH)2. 

Zirconium complexes have found worldwide use in increasing the flame resistance of wool (ZIRPRO process).24 The fabric is treated with a hot aqueous mineral acid solution containing ZrOCl2 and an a-hydroxy acid such as citric or tartaric acid. The nature of the species existing in the solution appears to be unknown, but a-hydroxycarboxylate chelates are thought to be involved.25... 

The absorption and emission spectra of Ru(bpy) + also showed pronounced spectral shifts (compared to that in water) upon intercalating into a sulfonated derivative of layered a-ZrP, zirconium sulfophenylphosphonate, or ZrPS [84], The XRD and the infrared (IR) spectra of the mixture, on the other hand, indicated that the chemical and structural characteristics of the complex were not significantly affected by the host matrix. The diffuse reflectance spectra, however, revealed three important differences from that observed in aqueous solution. These are that (1) the tt-tt band was strongly red-shifted to 317-320 nm from that observed at 285 nm in aqueous solution, (2) the MLCT band also red-shifted, depending on the loading, from 462 to 494 nm, and (c) the ratio of the intensity of the tt-tt band to the MLCT band decreased with loading (Fig. 40). 

Some physical properties and structural features of the normal tetramandelate have been reported. The solubility of the 1 4 mandelato complex in 2 Jlf perchloric acid was determined to be 7.8 x 10" mole/ liter (444), and was found to fall slowly with increasing pH to a minimum of 4 X 10 at pH 3.1, after which it then rose. The change in solubility was accompanied by a change in composition which involved the formation of metal oxo species. In 1958, R. W. Stromatt (540) concluded on the basis of infrared spectroscopy, that the tetramandelates exist as discrete 8-coordinate molecular species. This would seem to be supported by the fact (24) that an organic-soluble species can be extracted from aqueous solutions containing (1.0 to 8) x 10 mole/liter zirconium(IV) in 1 M perchloric acid with an isopentyl alcohol solution of p-bromo-mandelic acid. The normal tetramandelate precipitated from aqueous solution at the usual concentration conditions, however, is very insoluble... 

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