Carbonato complexes formation

After carbonato complex formation is complete, the solution is stirred for 5 min and cooled in an ice bath then absolute ethanol is added slowly until a precipitate appears. The mixture is allowed to stand near 0° for 1 hr, and... 

Ratios of U and U to Th and Ra daughters, combined with differences in chemical reactivity have been used to investigate the formation and weathering of limestone in karst soils of the Jura Mountains, and of the mountains in the central part of Switzerland. Uranium contained within calcite is released during weathering, and migrates as stable uranyl(VI) carbonato complexes through the soil. In contrast, the uranium decay products, Th and Ra,... 

The solubility of Tc02 nH20 in an aqueous solution was investigated by Meyer et al, [55-58], Paquette and Laurence [59] reported that carbonato- and phosphato-complex formation was likely to occur. The precipitation of Tc02-H20 is prevented in solution if the ratio... 

For the understanding of the binding of heavy metals on clays one needs to consider - in addition to ion exchange - the surface complex formation on end-standing functional OH-groups. Furthermore, the speciation of the sorbate ion (free hydroxo complex, carbonato- or organic complex) and its pH-dependence has to be known. 

The different types of coordination modes of carbonate in metal carbonato complexes offer challenges to investigate intimate mechanisms of their formation. The study of the kinetics and mechanisms of the formation and decomposition of such complexes, therefore, continue to be an interesting area of activity as it has a direct bearing on... 

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). 

The kinetics of the formation of carbonato complexes by CO2 uptake reaction has been extensively investigated (3). The two methods of study, the equilibrium method and acidification method , genuinely devised by Harris and co-workers were adapted to the stopped-flow kinetics measurement (63). The following general reaction scheme was proposed by them for the formation of monodentate carbonato/bicarbonato complexes by the C02 uptake reaction of M-OH(K-1)+ species (Scheme 2). 

The stopped-flow and T-jump techniques were used by Mao and co-workers (34j) to investigate the kinetics of complex formation of [Cu(tren)(OH2)]2+ by HC03. In large excess of HC03 and pH = 6-9, the formation of the mononuclear bicarbonato/carbonato complex was too fast to be studied kinetically. The kinetically observed reaction was the reversible formation of the binuclear complex, [(Cu(tren))2(02C0)]2+ and this occurred as shown below ... 

The latter two processes are slow as a result of which the detection of the protonated carbonato complex, and determination of its protonation constant by stopped-flow, rapid scan spectrophotometry were possible in some cases, van Eldik et al. (83) interpreted the initial spectral changes (shift of absorption maxima and the isosbestic points) of ( -cis-[Co(edda)C03l and [Co(nta)COa] at [H+]=0.05 M due to the formation of the protonated carbonato complexes, p-cis-[Co(edda)C03H] and [Co(nta)C03H], Repetitive rapid scan spectral measurements at [H + ] = 2.0 M, however, led them to the identification... 

A number of papers on some transition-metal model complexes have been reported in the literature to mimic the activity of CA and explore further information on the catalytic mechanism of CA. This aspect has been dealt with in Section IV on formation and aquation/decarboxylation of carbonato complexes. A few more relevant... 

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. 

In the presence of trace amounts of water, the tetrameric p,2-oxo complex (182) in 1,2-dimethoxyethane is transformed into a p, -oxo tetrameric complex (183 equation 254), characterized by an X-ray structure.574 In contrast, (182) 572,575 is inactive towards the oxidation of phenols. The reaction of N,N,N, AT -tetramethyl-l,3-propanediamine (TMP) with CuCl, C02 and dioxygen results in the quantitative formation of the /z-carbonato complex (184 equation 255).s76 This compound acts as an initiator for the oxidative coupling of phenols by 02. 6 Such jz-carbonato complexes, also prepared from the reaction of Cu(BPI)CO with 02 [BPI = 1,3 bis(2-(4-methyl-pyridyl)imino)isoindoline],577 are presumably involved as reactive intermediates in the oxidative carbonylation of methanol to dimethyl carbonate (see below).578 Upon reaction with methanol, the tetrameric complex (182 L = Py X = Cl) produces the bis(/z-methoxo) complex (185 equation 256), which has been characterized by an X-ray structure,579 and is reactive for the oxidatiye cleavage of pyrocatechol to muconic acid derivatives.580,581... 

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. 

To 27.5 g. (0.1 mole) of crude (carbonato)bis(ethylenediamine)-cobalt(III) chloride is added 200 ml. of 1.00 N hydrochloric acid. The carbonato complex is dissolved with evolution of carbon dioxide gas and formation of a red solution consisting primarily of the corresponding cw-diaqua species. The solution is evaporated in the steam bath until an almost dry paste has been formed. The purple residue is filtered and washed with three 20-ml. portions of ice-cold water. Drying in air yields 19.5 g. of purple crystals of cu-dichlorobis(ethylenediamine)cobalt(III) chloride. The mother liquor and the washings are again evaporated almost to dryness to yield a second crop of crystals, 5.9 g. The total yield is 25.4 g. (84% based on (carbonato)bis(ethylenediamine)cobalt(III) chloride). The analysis and the visible absorption spectrum of the two fractions are identical. Anal. Calcd. for [Co(en)2Cl2 ] C1 H20 Co, 19.42 N, 18.46 C, 15.82 Cl, 35.05 H, 5.98. Found Co, 19.50 N, 18.57 C, 15.77 C1, 35.15 H, 6.01. 

Formation of Carbon Dioxide Complexes. As mentioned in the introduction, our initial interest in synthesizing the PCy3 complexes was in their potential for binding C02. However, except for the formation of peroxycarbonate and carbonato complexes from IrCl(02)(PCy3)2 (44), which is well-established chemistry for some platinum metal peroxide complexes (42) (but, to our knowledge, not with PCy3 systems), we have not been able to isolate any C02 complexes or even carbonate or bicarbonate species which are formed sometimes in the presence of adventitious water (16). 

The previously unknown carbonato-complex [Ir(C03)(NH3)5]C104 has been isolated by treating [Ir(NH3)5(H20)](C104)3 with aqueous Li2C03 at pH 8.5, and the kinetics of its formation and aquation were studied.174 Stability constants have been measured using pH titration for a variety of o-coumaric acid complexes, including a 1 3 species with Ir111.74... 

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. 

Tetravalent. The best-studied tetravalent actinide carbon-ato complex is An(C03)5 (An = Th, U, Pu). This anion has been isolated using a variety of cations, including Na+, K+, T1+, [Co(NH3)6] + and C(NH2)3+/NH4+. In solution, the pentacarbonato complex is the end member of the series An(C03) " " (n = 1-5) however, in the mineral tuhokite, Na6BaTh(C03)6-6H20, thorium exists as a hexacarbonato complex. The analysis of the thermodynamic data for these actinide carbonate systems has led to differences of opinion on the actual speciation. The data appear to support both the stepwise addition of C03 and subsequent loss of H2O molecules within the An + cation coordination sphere as well as the formation of mixed hydroxo carbonato complexes, for example Pu(C03)3(0H) . 

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. 

Figure 10.19. Speciation of Pb(II) in the Glatt River. The concentrations given for CO2, Pb(II), Cu(II), and [Ca ] as well as for the pollutants EDTA and NTA are representative of concentrations encountered in this river. The speciation is calculated from the surface complex formation constants determined with the particles of the river and the stability constants of the hydroxo, carbonato, NTA, and EDTA complexes. The presence of [Ca " ] and [Cu " ] is considered. (From Muller and Sigg, 1990.)...

Carbonates and carboxylates. There is only one Am carbonato complex reported. From combined spectroscopy and cyclic voltammetry data in bicarbonate/carbonate solutions, it was concluded that Am(C03)5 is the limiting carbonate complex of Am. Its formation is consistent... 

IR photodissociation spectra were reported for the clusters Al+(C02)n and Al+(C02)n.Ar, and compared with the results of ab initio calculations (all in the region of vasC02 modes).378 The complex (68), where R = 2,4,6-Ph3C6H2, has vCO at 1624 cm-1, compared to 1696 cm-1 in the free ligand, confirming the coordination shown.379 There is IR evidence for the formation of bi- and poly-dentate carbonato complexes by the adsorption of gaseous C02 on to (t-Ga203.380... 

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