Uranyl thiocyanate

Uranyl Thiocyanate, U02(CNS)2.8H30, may be obtained by the addition of barium thiocyanate to a solution of uranyl sulphate, and evaporation of the resulting red solution in a vacuum over sulphuric acid. It separates in orange-yellow needles, which are very hygroscopic and readily dissolve in water, alcohol, ether, and acetone ether may be used for extracting it from its aqueous solution. The addition of jyridine to its aqueous solution precipitates a crystalline basic salt of composition 5U02(CNS)2.U03. rH20. 

Table 21.15 Thermodynamics of uranyl thiocyanate for-mation in 1 m NaClO, at 25°C [94].

The extraction of uranyl thiocyanate by diethyl ether at various Initial NH j SCN and HCl concentrations. 

AqueouB thiocYamte B2BtemB Vanoasl- has deocrlbed a procedure In which uranium la extracted by ethyl acetate from an aqueous phase containing an excess of ammonium thiocyanate. Dlzdar and Obrenovlo Xi have also Investigated the extraction of the uranyl-thiocyanate complex by ethyl acetate. 

The majority of U(V1) coordination chemistry has been explored with the trans-ddo s.o uranyl cation, UO " 2- The simplest complexes are ammonia adducts, of importance because of the ease of their synthesis and their versatihty as starting materials for other complexes. In addition to ammonia, many of the ligand types mentioned ia the iatroduction have been complexed with U(V1) and usually have coordination numbers of either 6 or 8. As a result of these coordination environments a majority of the complexes have an octahedral or hexagonal bipyramidal coordination environment. Examples iuclude U02X2L (X = hahde, OR, NO3, RCO2, L = NH3, primary, secondary, and tertiary amines, py n = 2-4), U02(N03)2L (L = en, diamiaobenzene n = 1, 2). The use of thiocyanates has lead to the isolation of typically 6 or 8 coordinate neutral and anionic species, ie, [U02(NCS)J j)/H20 (x = 2-5). 

Sodium aluminium sulphate Sodium bisulphate Sodium hypochlorite Sodium perchlorate Sodium thiocyanate Stannic ammonium chloride Stannic chloride Stannous chloride Uranyl nitrate Zinc chloride Zinc fluorosilicate... 

At higher temperatures the monomer is the predominant species although the rate of hydrolysis to U03 is increased. U03 dissolves in uranyl solutions to give U02OH+ and polymerised hydroxo-bridged species. Polynuclear species could arise from U4+ as it hydrolyses in dilute acid solutions. Complex ions are formed with thiocyanate, phosphate, citrate and anions of other organic acids. 

Inorganic extractions seem to have come into practical use without any great notice. Although Peligot in 1842 reported that uranyl nitrate could be recrystalhzed from ether, he never mentioned extraction of this salt from aqueous solutions. In textbooks after 1870, however, it is stated briefly that ether can even withdraw sublimate (HgCy from aqueous solution. It was also reported, for example, that cobalt thiocyanate is weakly extracted by ether, better by amyl alcohol, and even better by a mixture of both. 

Solubility in Aqueous Solutions of Ammonium Sulfate, Ammonium Acid Sulfate, Sodium Chloride, Sodium Nitrate, Lead Nitrate, Uranyl Nitrate and Ammonium Thiocyanate... 

Hexavalent. The majority of An(VI) coordination chemistry with N-donors has been explored with the uranyl cation, 50i. Stable adducts with the hgands discussed in the tri- and tetravalent complexes have been described, for example, U02X2L (X = halide, OR, NO3, RCO2). The coordination numbers observed for these complexes are typically 6, 7, or 8 with octahedral, pentagonal bipyramidal, or hexagonal bipyramidal geometries, respectively. Neutral and anionic thiocyanates have also been isolated, for example [U02(NCS)j2- yH20(x = 2 5). 

The compounds (Et4N)4[M(NSCe)a] (M = Pa, U) have been characterized by comparing their spectral and crystallographic properties with the corresponding thiocyanate complexes (26). Some thorium(IV) complexes have been prepared containing DMF and N-bonded seleno-cyanate groups (326). Although various uranyl selenocyanate complexes have been reported, no structural data are available (686). 

Thiocyanate reacts with uranyl ions in acid medium to form a series of yellow complexes such as U02SCN, U02(SCN)2, and U02(SCN)3. Higher concentrations of thiocyanate displace the equilibrium towards the last-mentioned and more intensely coloured complex. The absorption maximum of this complex lies in the near ultraviolet, at -350 nm. At wavelengths shorter than 360 nm thiocyanate ions begin to absorb. 

The vast majority of the macrocyclic complexes of the lanthanide(III), yttrium(III) and uranyl ions obtained so far by metal-templated synthesis are of the Schiff-base type even the few known examples of simple polyamine complexes actually result from the cyclic condensation of a diamine with a (modified) carbonyl precursor. In general, the metal-templated synthesis is facilitated by the presence of oxygen-donor anions, such as nitrate, acetate, or trifluoromethylsulfonate lanthanide(III) thiocyanates have also been successfully used. With a few exceptions, the outcome of the synthesis appears to be independent of the order of addition of the reactants. No deliberate attempts have been made to investigate the detailed mechanism of these metal-templated cyclic condensation reactions. 

Ramkumar, J., Maiti, B., Mathur, P.K. Dhole, K. (2000) Crown ethers as carriers for the transport of anionic thiocyanate complex of uranyl ion across a bulk liquid membrane. Separation Science and Technology, 35(15), 2535-2541. 

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