Oxalate and malonate complexes

Che and Kustin studied complexation 438 results for oxalic and malonic acids are in Table 27. From previous relaxation data 07 and their own results, they concluded that the rate constants are more consistent with a normal dissociative pathway if VOL formation from [VO(OH)]+ is assumed. 

Many formation constants involve polycarboxylates Table 28 summarizes the data. Nagyp l and Fabian s report on the oxalic and malonic systems seems the most complete as hydrolysis of both metal ion and complexes has been included.584 A concentration distribution of the complexes in the malonic system is shown in Figure 25. The order of basicities is succinic > citraconic > itaconic > maleic > malonic acid and log /3U0 should follow the same order. However, from Table 28, the order of stabilities is citraconic > malonic > maleic > itaconic > succinic acid.608... 

The effect of exchange of lactic, mandelic and sulfosalicylic acids on the relaxation of solvent protons gave rate constants (k) of exchange from 1.73 to 0.701 mol-1 s-1.642 Kinetics of complex formation with mandelic (HMDA) and vanillomandelic acids (HVMDA) gave rate constants (1.09 x 103 and 1.13 x 103 mol-1 s 1 for MDA- and VMDA ) consistent with a dissociative (Eigen) mechanism.438 As in the case of oxalic and malonic acids (Section 33.5.5.5.ii Table 27), species with coordinated hydroxyl are labilized. 

Hydrolysis of chelated oxalate and malonate is very slow and both systems can be considered inert at ambient temperatures. There is no report of or M"+-catalyzed ring opening there is no easy point of attachment of the catalyst in such systems. Acid hydrolysis of monodentate complexes at elevated temperatures (Table 59) often follows the rate law = ki + with fcj correspond-... 

Oxalic and malonic acids, as well as a-hydroxy acids, easily react with cerium(IV) salts (Sheldon and Kochi, 1968). Simple alkanoic acids are much more resistant to attack by cerium(IV) salts. However, silver(I) salts catalyze the thermal decarboxylation of alkanoic acids by ammonium hexanitratocerate(IV) (Nagori et al., 1981). Cerium(IV) carboxylates can be decomposed by either a thermal or a photochemical reaction (Sheldon and Kochi, 1968). Alkyl radicals are released by the decarboxylation reaction, which yields alkanes, alkenes, esters and carbon dioxide. The oxidation of substituted benzilic acids by cerium(IV) salts affords the corresponding benzilic acids in quantitative yield (scheme 19) (Hanna and Sarac, 1977). Trahanovsky and coworkers reported that phenylacetic acid is decarboxylated by reaction with ammonium hexanitratocerate(IV) in aqueous acetonitrile containing nitric acid (Trahanovsky et al., 1974). The reaction products are benzyl alcohol, benzaldehyde, benzyl nitrate and carbon dioxide. The reaction is also applicable to substituted phenylacetic acids. The decarboxylation is a one-electron process and radicals are formed as intermediates. The rate-determining step is the decomposition of the phenylacetic acid/cerium(IV) complex into a benzyl radical and carbon dioxide. 

Giordano (1990) calculated concentrations of Ca, Mg, Na, Pb, Zn, Fe, and Al as acetate, oxalate, malonate, succinate, and catechol complexes in three reconstructed MVT ore solutions and a model oil-field brine at 100 °C. In Table 5, results are presented for revised speciation calculations for the three MVT ore fluids and calculated speciation results for a model RBRBM ore fluid. These model results are based in part on recently published data at elevated temperatures for Ca, Mg, and Al complexes and high-temperature constants for ionization reactions of oxalic and malonic acids (Table 4). In the MVT model proposed by Anderson (1975 model 1, Table 5), the ore fluid is moderately oxidized and falls well above the sulfide-sulfate boundary in logao2 - pH space at 100°C. It is slightly acid and contains 10 molal... 

The complexes formed by oxalic and malonic acids have been carefully studied and the detailed structural features have been obtained from X-ray diffraction measurements (Hansson, I973e). The oxalate ions in these complexes serve as bridging ligands and the larger metal ions have coordination number nine whereas the smaller ions have coordination number eight. The malonate ions in the rare earth malonates are of two different types, those that are involved in six-membered chelate ring formation and those which are not. 

Effect of Chelate Ring Size on Complex Stability with Metal Ions of Different Sizes Oxalate (Ring Size Five) and Malonate (Ring Size Six)°... 

Simple organic molecules such as small carboxylic acids (oxalate, acetate, malonate, citrate, etc.), amino acids and phenols are all ligands for metals. Such compounds may all occur as degradation products of organic matter in natural waters. The complexes formed are typically charged hydrophilic complexes. The stability of the metal complexes with these ligands is, however, moderate in most cases. Model calculations including such compounds at realistic concentrations indicate that their effects on speciation are relatively small [29],... 

As the size of the chelating ligand increases, a maximum in stability is normally obtained for 5 or 6 membered rings. For lanthanide complexes, oxalate forms a 5-membered ring and is more stable than the malonate complexes with 6-membered rings. In turn, the latter are more stable than the 7-membered chelate rings formed by succinate anions. 

Malonic acid CH2(C02H)2 (H2mal) (209) has a coordination chemistry with chrommm(III) closely resembling that of oxalate. Malonic acid is a slightly weaker acid than oxalic acid and slightly more labile complexes are formed. The tris complex is the most extensively studied, prepared by the reduction of chromate solutions or the reaction of chromium(III) hydroxide with malonate.917,918 919 The cis and trans diaqua complexes may be prepared by the reduction of chromate with malonate the isomers are separated by fractional crystallization. The electronic spectrum of the tris complex is similar to that of the tris oxalate and a detailed analysis of these spectra has appeared.889... 

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