Polynuclear complexes uranium

The IR spectra suggest that the uranium is indeed bound to the ring oxygens. The asymmetric stretch frequency associated with the CH2CH2OCH2CH2 system shifts to lower frequency by about 20-40 cm i in the complexes 81). It has been suggested that the polynuclear complexes are chlorine bridged, 4. 

As hydrolysis changes the light absorption very markedly, and because precise potentiometric studies are difficult at high acidities, it was natural to use spectrophotometric methods in the first investigations of the hydrolysis [39-41]. At low uranium concentrations and high acidities, where the first mononuclear species UOH " predominates, such methods work weU. At lower acidities, polynuclear complexes exist, however, and for their investigation potentiometric measurements have been performed [42]. Values of p f, are listed in Table 21.2. 

In the potentiometric study [42], both the acidity and the uranium concentration Cm were varied within wide limits, and both [H" ] and [U ] were measured, the latter via the iO fVi couple. The existence of the mononuclear UOH was confirmed. Polynuclear complexes predominate in the later stages of the hydrolysis, the more so the higher the value of Cm> A series of core -I- links complexes U((OH)3U)J was presumed [42]. It has since been pointed out, however, that the core-1-links hypothesis has been discredited in several other cases, among them the closely related Th [25]. Except for the highest ligand numbers, the data may in fact be described by the monomer UOH and only one of the polymers previously postulated, viz. the hexamer. ... 

The formation of very large polymers, and the ensuing precipitation, takes place at much lower ligand numbers for uranium(iv) than for thorium(iv). It is therefore much more difficult to study the structures of the hydroxo complexes of uranium(iv) in solution. Polynuclear complexes are formed, with U-U distances... 

Although stable, homoleptic, mononuclear and polynuclear complexes of carbon monoxide are known for almost every transition element, a number of experiments indicate that actinide carbonyls (e.g. U(CO) ) are not stable at ambient temperature. However, uranium carbonyls can be prepared in cryogenic matrices [6,7]. Co-condensation of uranium vapor with CO in a 4K argon matrix yields species identifiable by infrared spectroscopy (in particular, Voo) uranium carbonyls, U(CO) . Derived force constants suggest bonding patterns rather similar to those observed for zero-valent early transition-element and lanthanide carbonyls. However, decomposition of these actinide carbonyls occurs at temperatures above about 30 K. 

Berthet, J.C., Nierlich, M., Miquel, Y., Madic, C., Ephritikhine, M. 2005. Selective complexation of uranium(III) over lanthanide(IH) triflates by 2,2 6, 2"-terpy ridine. X-ray crystal structures of [MfOTf fterpy ] and [M(OTf)2(terpy)2(py)][OTf](M = Nd, Ce, U) and of polynuclear (x-oxo uranium(IV) complexes resulting from hydrolysis. Journal of the Chemical Society, Dalton Transactions 2 369-379. 

As with uranium, the solution chemistry is complicated, owing to hydrolysis and polynuclear ion formation, complex formation with anions other than perchlorate, and disproportionation reactions of some oxidation states. The tendency of ions to displace a proton from water increases with increasing charge and decreasing ion radius, so that the tendency to hydrolysis increases in the same order for each oxidation state, th at is, Am > Pu > Np > U and M4+ > M02+ > M3+ > M02 simple ions such as Np02OH+ or PuOH3+ are known in addition to polymeric species that in the case of plutonium can have molecular weights up to 1010. 

When alkali metal bases are used to raise the solution pH to moderate levels, the uranium will precipitate from the solution in the form of hydrous uranyl hydroxides or uranates, for example, Na2U207. However, through judicious choice of a base, for example, tetramethylammonium hydroxide, (TMA)OH, or tetramethylaimnoirium trifluoromethansulfonate, the study of the amphoteric behavior of uranyl hydroxides can be undertaken. Polynuclear anions of the form (U02)3(0H)7, (U02)3(0H)g, and (U02)3(OH)io are examples of soluble species in solutions where the pH < 14. When the concentration of the (TMA)OH is increased (>0.6 M OH ), highly soluble ( 0.1M) monomers ofthe form U02(0H) "(n = 3, 4,5) have been reported. These three species are in equilibrium with each other however, in solutions where the [OH ] is greater that 1M, the pentahydroxo complex predominates the speciation. 

As stated previously, the above approach to solvent extraction is a simplified version. It represents only an average extracted species. Among other things. It does not consider the effect of water activity In the organic phase, solvent activity In the aqueous phase, complex formation between the various components In either phase, or the formation of polynuclear species. These effects may be large or small depending upon the solvent, aqueous medliim, and uranium concentration Involved. 

Davydov, Y.P. and Efremenkov, V.M. (1975) Investigation of the hydrolytic properties of tetravalent uranium. II. Conditions of formation of mononuclear and polynuclear hydroxo complexes of U(IV) in solution. Radiokhimiya, 17, 160-164. 

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