Carbonato complex ions

Addition of ammonium carbonate to a solution containing an actinide(III), (IV),(V) or (VI) ion gives the following results. Only actinide(VI) ions form soluble carbonato complex ions. Actinide(III) and (IV) ions precipitate as their hydroxides or basic carbonates, and actinide(V) ion precipitates as a double carbonate. Therefore, in dilute ammonium carbonate medium, U(VI) ion can be separated primarily from Np(V), Pu(IV), Am(III) and Cm(III) ions. Further addition of ammonium carbonate leads to complex ion formation and the dissolution of actinide(IV) precipitates. However, most of the actinide(III) and (V) ions remain as precipitates under this condition. Crystalline precipitates of actinide(IV) and (VI) carbonato complex anions are formed by addition of hexamminecobalt(III), hexaureachromium(III) or hexa-mminechromium(III) salt to the ammonium carbonate solution containing actinide(IV) and (VI) ions. 

Figure 2 shows the precipitation behavior of Py(IV) and U(VI) carbonato complex ions. The concentration of Pu(IV) ions in the supernatant decreases gradually to a minimum ( 20jjgPu/mL) in the range of 0.15 to 0.3M (NH.)2C03 when the concentration of hexamminecobalt(III) chloride is4kept at 0.04M. The Pu(IV) concentration increases, then, with increasing concentration of am-... 

Actinide(III) precipitates dissolve to a limited extent in solution containing 1M (or more) (NH J CO. However, the actinide (I I I) carbonato complex ion precipitates very slowly by adding hexamminecobalt(III) chloride and the yield of precipitation is not high. Separations of actinide(IV) and (VI) ions from actinide (III) and (V) ions are thus achieved by taking advantage of their different solubilities in ammonium carbonate solution. Hexamminecobalt (III) salt is used as a precipitant to recover U(VI), Pu(IV) and Am(VI) ions from ammonium carbonate solution. 

Ion Excha.nge, The recovery of uranium from leach solutions using ion exchange is a very important process (42). The uranium(VI) is selectively adsorbed to an anion-exchange resin as either the anionic sulfato or carbonato complexes. In carbonate solutions, the uranyl species is thought to be the tris carbonato complex, U02(C03) 3 [24646-13-7] and from sulfate solutions the anion is likely to be U02(S0 , where nis ) [56959-61-6] or 2 [27190-85-8], The uranium is eluted from the resin with a salt or acid solution of 1 AfMCl or MNO (M = H", Na", The sulfate solution is... 

The product CO3 radical can react with a variety of inorganic ions including the carbonato complex of Pu(V). Thus, in a pulse radiolysis experiment, Pu(VI) in 0.05 M Na2C03, He saturated, there are the parallel reactions (17) and (18)... 

Each entry has the % of total metal present as the free hydrated ion, then the ligands forming complexes, in decreasing order of expected concentration. For instance, in inorganic freshwater at pH 9, Ag is present as the free aquo ion (65%), chloro-complexes (25%), and carbonato-complexes (9%). 

An analogous nitrato complex, Be40(N03)6, is known in the solid state (145). The presence of a central 4-coordinate oxide ion and bridging nitrato ions has been confirmed by X-ray methods (146). The structures of these solid-state (jl4-oxo complexes lend support to the fi.roxo structures suggested for phosphato and carbonato complexes in solution (see Section IV). 

The stability constants of several carbonato complexes of metal ions in solution has been compiled (3). Several recent publications have dealt with the isolation and characterization of carbonato complexes... 

The carbonato complexes of lanthanides and actinides are of importance with regard to the metal ion speciation in the environment. These are, however, not linked with the enzyme models for carbon dioxide hydration and hence are not dealt with in further detail. 

The p3-bridged carbonato complex posseses a pseudo-3-fold molecular symmetry. Each of the Cu atoms is five coordinate with the four nitrogen atoms of tren and one oxygen atom of the carbonate ligand (C). The coordination polyhedron of the Cu atom can be described as almost (TBP), the copper ions being slightly out of the plane (0.15 A) of the three primary amine groups (Fig. 5). 

The hydration rate constant of C02, the dehydration rate constant of carbonic acid (H2C03), and p pK2 values (pTf, =6.03, pTf2 = 9.8 at 25 °C, 7=0.5 M) (63) are such that nearly 99% of dissolved carbon dioxide in water at pH < 4 exists as C02. However, these four different species may be considered as the reactive species under different pH conditions which can react with aqua metal ions or their hydroxide analogues to generate the metal carbonato complexes. The metal bound aqua ligand is a substantially stronger acid than bulk H20 ( )K= 15.7). Typical value of the p of H20 bound to a metal ion may be taken to be 7. Hence the substantial fraction of such an aqua metal ion will exist as M-OH(aq)(ra 1) + species at nearly neutral pH in aqueous medium. A major reaction for the formation of carbonato complex, therefore, will involve pH controlled C02 uptake by the M-OH(" 1)+ as given in Eq. (17). 

Carbonate has proved to be a versatile ligand. Its coordination mode expands from 1 to 6, the last one is visualized when each atom binds two metal ions simultaneously. In recent years, significant progress has been made in the synthesis, structure, and magnetic properties of polynuclear carbonato complexes of transition metal ions. Such studies have also been extended to lanthanides and actinides. The speciation studies of these metal ions in aquatic environments in the presence of carbonate have resulted in significant... 

Ceric ammonium nitrate, the well-known volumetric standard oxidant, is formulated (NH4)2[Ce(N03)6] with 12-coordinated Ce4+ ions (each N03 ion being bidentate) in approximately Td symmetry. The Ce—O distances are in the range 2.488-2.530 A. The ligand bite is only 50.6-50.9° and the other O—Ce—O angles are 65.0-69.90.692 The carbonato complexes Na6[Ce(C03)5]-12H20 and [C(NH2)3]6[Ce(C03)5]-4H20 each show cerium ten-coordinated to five bidentate carbonate ions with Ce—O = 2.379-2.504 A in the first complex and 2.388-2.488 A in the second. The coordination polyhedra are irregular.693,694... 

A number of basic oxalates, such as (U02)2C204(0H)2-2H20, and salts of basic oxalato complex ions of the types M3UO QO fOH) and Ms(U02)2(C204)4(0H) have also been reported, as well as salts of a wide range of peroxo-, halogeno-, sulfito-, sulfato-, selenito-, selenato-, thiocyanato- and carbonato-oxalato complex anions all need further investigation. 

Uranyl ion forms complexes with many oxy anions. Both U(VI) and U(IV) compounds dissolve in alkali carbonate solutions with formation of carbonato complexes. Those of the larger alkali cations are only slightly soluble KSp = 6 x 10-5 for both K4[U0 tC03)3] and iNHu tUOitCO, ] 2H 0. 

The following procedure is based on the reaction of an aqueous solution of cobalt(II) chloride with the equivalent amount of (2-aminoethyl)carbamic acid, followed by oxidation with hydrogen peroxide and the subsequent formation of bis(ethylene-diamine)cobalt(III) ions. The bis(ethylenediamine)cobalt(lII) species are converted to the carbonato complex by reaction with lithium hydroxide and carbon dioxide. During the entire preparation a vigorous stream of carbon dioxide is bubbled through the reaction mixture. This procedure appears to be essential in order to minimize the formation of tris(ethylenediamine)cobalt(III) chloride as a by-product. However, the formation of a negligible amount of the tris salt cannot be avoided. The crude salts have a purity suitable for preparative purposes. The pure salts are obtained by recrystallization from aqueous solution. 

Uranyl carbonato complexes are important and much studied since they occur in Nature and are of environmental concern this is also the case for other actinide carbonate complexes.40 The C03 ion is exceptionally strongly bound to UO + and similar actinide ions. There are several naturally occurring minerals such as U02C03 while the anion [U02(C03)3]4 has importance in the extraction of U from ores and is responsible for the migration of U02+ in ground waters. Interaction of the 4-ion with HC104 proceeds as follows ... 

From equilibrium studies it is known that the major cations (Na+, K+, Ca2+, and Mg2+, and the anion Cl-) are mostly present in their non-complexed form, but other minor elements (at <1 ppm) primarily exist as complex ions or ion pairs. The metal cations predominantly form hydroxo complexes and, to a smaller extent, carbonato and chloro complexes. 

If the numbers were accepted at face value, the data indicate that the species involved account for S0-S % of the toxicity, the carbonato complexes do not contribute to toxicity, the anionic hydroxo copper complexes contribute 13-18 to the total toxicity, and the free copper ion and/or the neutral and cationic hydroxo complexes of copper are responsible for 60-70 of the toxicity of copper to aquatic life. 

Complcxation of the actinides by inorganic and organic ligands is very important for their chemical. separation and their behaviour in the environment. Examples arc the separation of trivalent actinides by complcxation with a-hydroxy acids and the high solubility of U(VI) due to complcxation with carbonate ions to give the tris-carbonato complex U02(C02)3 " found in seawater. 

The logarithms of the stability constants fly for the formation of 1 1 complexes of the actinide ions M +, M " ", MOj and MO with various inorganic ligands are plotted in Fig. 21.1. Carbonato complexes of alkaline-earth elements, lanthanides, actinides and other transition elements play an important role in natural waters and may stabilize oxidation states. 

The ligated carbonate ion is easily released by the treatment with aqueous base. Thus, the reaction of complex 3 (5mM) in acetonitrile with 0.1 N NaOH aqueous solution afforded the complex 2, bis(hydroxy) Ni(II) complex quantitatively and instantaneously [9]. Half of the complex 3 was converted to complex 2 within a minute by the addition of 0.01 N NaOH aqueous solution to the 5mM complex 3 in acetonitrile [9]. As the complex 2 can reacts with CO2 and formed carbonato complex 3 can release the carbonate ion in the presence of aqueous base, it is expected that the catalytic CO2 hydration system can be developed by use of complex 2. 

In summary, the new bis(hydroxo) dinuclear Ni(II) complex was synthesized. This complex showed high reactivity toward atmospheric CO2, and the formed carbonato complex could release the carbonate ion easily by the treatment of aqueous base. 

A large majority of diacidobis(l, 2-ethanediamine)cobalt(III) complexes can be synthesized from the (carbonato)bis(l, 2-ethanediamine)cobalt(III) ion. This is now readily available in high yield and purity.1 cis- and trans-Dichlorobis(l, 2-ethanediamine)cobalt(III) chlorides are obtained from the carbonato complex with hydrochloric acid using appropriate conditions. These isomeric dichloro salts are widely used starting materials for further syntheses. In this respect, the analogous dibromo complexes can be more convenient starting materials because bromide is substituted from the cobalt center more readily than is chloride ion. The milder conditions minimize the production of cobalt(II), a common side reaction. 

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