Acid ligands

If an electrode surface comprises a film containing a weak organic ligand acid, HL, and if the aqueous phase contains ions M"+ which can form an extractable metal chelate ML , the electrode potential will be given by... 

The difference in acid strength of the cis and trans isomers of (NH3)5Cr(0H)Cr(NH3)4(H20)5+ may be explained as an effect of the ligand trans to water. A comparison with the effect of trans ligand on the water ligand acidity observed for mononuclear ammine and amine complexes of chromium(III) or rhodium(III) (346) leads to the sequence Hi OH) < OH NH3 < H20 for increasing acidity due to the trans ligand. 

The mechanism can only be retained if a base very much better than water is present at the enzyme active site to deprotonate the Zn(OH2)2+ species. In fact, the structure determination reveals a histidine residue with its side chain positioned approximately halfway down the -15 A-deep cleft in the protein structure within which the Zn site is located. This arrangement could act as a proton shuttle between the Zn(OH2)2+ and external solvent water, possibly via another two water molecules also found within the cleft. As a consequence, the enhancement of ligand acidity by Zn11 is more important in the kinetic than the equilibrium sense (taken from http //www.chem.uwa.edu.aU/enrolled students/BIC sect4/sect4.2.htmll. 

Zhou et al. studied the extraction of alkaline earth metal cations with p-tert-calix[4]arene-l,2-crown-5 under different conformations. The pH for half loading, pH0 5, is a qualitative measure of ligand acidity. The pH0 5 values for a given conformation decrease as the electron-withdrawing ability of X increases. Moreover, the ligand acidity increases uniformly as the conformation is varied in the order 1,3-alternate < partial-cone < cone. This order is related to the proximity of the ionized groups to the complexed cations.140... 

Fig. 8.10. Plot of oscillator strength P of the 1 1 complexes of Ho3 with monobasic (+) and dibasic ( ) ligands vs. the pXa of the ligand acid (from ref. [741) (a) fluoride, (b) glycolate, (c) acetate, (d) propionate, (e) of-picolinate, (f) tropolonate, (g) kojate, (h) acetylacetonate, (I) IMAD, (2) dipicolinate, (3) methyl succinate, (4) malonate, (5) maleate, (6) fumarate, (7) sulfate.

CAimHPFd with imidazolium tag on the diamine ligand acid/NEt3 azeotrope as hydrogen source catalyst recycled four times ee > 90%. ... 

Many organic ligands are the conjugate bases of comparatively weak acids and their metal stability constants are readily obtained by pH-metric titrations after a preliminary determination of the important hydrogen-ion stability constants of the ligand acid itself... 

The dependence of ligand acidity on the metal oxidation state has been well documented in particular by electrochemical studies which have shown, e.g., a decrease of pof ca. 7 or 10, in aprotic media, for the ligated isocyanide or aminocarbyne CNHX (x = 1 or 2, respectively) at phosphinic Fe or Re centers,113,114 upon single-electron metal oxidation, whereas even more dramatic pKa decreases, up to 20-30, have been reported for hydride ligands,115-117 the processes leading to proton release. 

The water-insoluble ammonium ligands are prepared from available water-soluble sodium salts according to Eq. (1). By introducing different amines in solvents, the sodium cations of the sulfonates can be exchanged in the presence of sulfuric acid. Sodium hydrogensulfate as a by-product can be removed smoothly by phase separation. This re-immobilization technique may also be extremely useful in the separation of salts from raw sulfonation mixtures within the preparation of sulfonated phosphines [12]. According to Eq. (1) many variations are possible, for example in the amines, functionalized phosphorus ligands, acids, solvents, and preparation procedures. The variability of the monoamines is demonstrated in Table 1. 

Ligand Acidic extraction Rhodium content [fig] Rh recovery %] Rh RAd>... 

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