Oxalate anion

After preparing a homogeneous solution of the precursors, powder precipitation is accompHshed through the addition of at least one complexing ion. For PLZT, frequently OH in the form of ammonium hydroxide is added as the complexing anion, which results in the formation of an amorphous, insoluble PLZT-hydroxide. Other complexing species that are commonly used are carbonate and oxalate anions. CO2 gas is used to form carbonates. Irrespective of the complexing anion, the precipitated powders are eventually converted to the desired crystalline oxide phase by low temperature heat treatment. 

In this case the parameters C and Q are of order of unity, and therefore they correspond to the intermediate situation between the sudden and adiabatic tunneling regimes. Examples are mal-onaldehyde, tropolon and its derivatives, and the hydrogen-oxalate anion discussed above. For intermolecular transfer, corresponding to a weak hydrogen bond, the parameters C, Q and b are typically much smaller than unity, and the sudden approximation is valid. In particular, carbonic acids fulfill this condition, as was illustrated by Makri and Miller [1989]. 

As seen in the preceding chapters, macrocyclic polyamines such as [18]aneN6 can be cation chelators, anion chelators, or both depending on the conditions. It was expected that with these dual properties, the macrocyclic polyamines might serve as litholytic agents by removing Ca2+ and phosphate or oxalate anions from insoluble calculi. 

EDTA, leading to a postulate that more than one equivalent of Ca2+ can be captured by X (e.g. one Ca2+ sequestered by the three amines and the three carboxylates and another Ca2 + by the remaining half the donor groups), as the Dreiding model suggests. The fact that there was no interaction at neutral pH of X with phosphate or oxalate anions was separately confirmed. Thus, the dissolution of Ca3(P04)2 and Ca(C204) is entirely due to the cation complexation mechanism. 

Comparison of Ca3(POi)2, Ca(C204), and Mg3(P04)2. The effects of cation and anion composites were tested by comparing the dissolution of Ca3(P04)2, Ca(C204), andMg3(P04)2 at pH 7 (Table 10). The dissolution of Ca3(P04)2 is achieved more effectively with X than with EDTA. However, when the anion is oxalate, the dissolution of Ca2+ is drastically reduced with X. EDTA can dissolve Ga(C204) to the same extent as Ca3(P04)2, i.e. the anion effect is insignificant. A better separation of Ca2+ from oxalate anion may be achieved by EDTA. 

Some ligands have more than one atom with an unshared pair of electrons and hence can form more than one bond with a central metal atom. Ligands of this type are referred to as chelating agents the complexes formed are referred to as chelates (from the Greek chela, crab s claw). Two of the most common chelating agents are the oxalate anion (abbreviated ox) and the ethylenediamine molecule (abbreviated en), whose Lewis structures are... 

It may not be that surprising that an effective homogeneous catalyst for the reaction shown by Eq. (4) has not been found it is difficult to imagine a facile mechanism by which oxalate anion or oxalic acid could be generated at a metal center. 

Table 2 summarizes the numerical results on average intermolecular/interi-onic 0---0 and intramolecular/intra-ionic C-0 structural parameters for the [COOH]n---[COOH]n, [COOH]a---[COOH]a and [COOH]A---[COO]A inter-molecular interactions, together with those obtained for the [COOH] A- [COO ]A sample in the case of the hydrogen oxalate anion. Data were retrieved from the CSD with a cut-off distance on 0-"0 separations of 3.0 A. A visual prospect of the data listed in column III is provided in Fig. 6, where histograms of intramolecular C-0 distances within the protonated and deprotonated COOH/COO groups are presented. 

Under certain conditions, oxalic acid (let L represent the oxalate anion) can form soluble complexes with calcium and iron ions. 

If Cl represents the total concentration of oxalate anion, then... 

Let s imagine that several studies (7-9) suggest that chromium oxalate cannot enter bacteria cells because of its size. (It is too big the oxalate anion is -OOC-COO , significantly larger than either NOj or Cl .) Add a sentence or two to the end of the second paragraph (P3) to relate the current work to these studies. 

The EPR spectral data indicated that two cyanide anions bind to copper at low temperature where two cyanide anions and two histidines are present in the basal plane and the third histidine residue is present in the axial position. It has been proposed that the second cyanide anion displaces the coordinated water. Similarly, it has been proposed that the oxalate anion coordinated in a bidentate fashion and displaced the coordinated water. In case of sulfonamides, the coordination geometry is reported to be the same as that of ZnCa. 13C NMR spectroscopy was used to explore the location of C02 and HCO3 with respect to metal ion in CuCa (129,130,138). It indicated that HCO3 is bound directly to Cu (137). The affinity constant of C02 for CuCa is <1 M-1 but the paramagnetic effect is paradoxically high (130). These results indicated that C02 does not bind to a specific site but probably is attracted by the cavity either by hydrophobic interactions or by the metal ion or by both. 

Such behavior is called an adsorption envelope .13 The adsorption of oxalate anions on silica is minimal, because silicon dioxide has mainly negatively charged sites in the measured pH range (from 3.0 to 10.0). Oxalate ions do not adsorb nonspecifically on such an interface. 

The adsorption of oxalate anions onto metal oxide surfaces changes the C, potential in the system. This is shown by data in Figure 6. As one can see, the C, potential for the goethite /electrolyte solution system in the presence of oxalates has negative values in the entire measured pH range for the highest oxalate ions concentration (0.001 M), and decreases with an increase of oxalate ion concentration. 

Oxalate anions can form with Cd2+ cations two types of complexes CdC204 and Cd(C204)2-. The main complexed form in the measured pH range is CdC204. 

II has a two-dimensional structure with an asymmetric unit of 16.75 non-hydrogen atoms (Figure 5a). The Pb2+ cations are in three crystallographically distinct positions with Pb(l) and Pb(3) landing with 0.5 occupancies in 4fand 4h special positions, respectively, and Pb(2) with a full occupancy. One CHDC anion, one-quarter of the oxalate anion (with C at 4h), one hydroxyl anion (with the O at 4/), one independent oxo dianion (at 4/), and one-quarter of a lattice water molecule (at 2a) are also in the asymmetric unit. Three of the four anions are shown in Figure lc—e. The CHDC anion in the anti, e,e conformation with a torsional angle of 176.74(2)° has (2223) connectivity and binds to six Pb2+ cations [three Pb(2) and three Pb(3)]. The oxalate anion has (2222) connectivity and binds to six Pb2+ cations [two Pb-... 

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