Metal cation-hydroxy complexes

Figure 3.5 Plot of the association constant of some 1 1 metal cation-hydroxy complexes at zero ionic strength (see Chap. 4) versus the electrostatic function luZon/irti + Toh). where the association reaction is written Af + OH"=A/OH " , and z and r are the charge and radius in nanometers (nm) or angstroms (A) (1 nm = 1 A) of cation M and OH ( oh = 1-40 nm). Cation radii are from Shannon and Prewitt (1969), log values from Baes and Mesmer (1981). The slope of the straight line suggests the contribution of electrostatic (ionic) bonding to the stability of the complexes. The extent to which species plot above this line presumably reflects the increased contribution of covalency to their stabilities.

The original pillared clays were made by (/) mixing smectite with a polymeric cationic hydroxy metal complex such as aluminum chlorhydrol (2) allowing a minimal amount of time for the cationic hydroxy metal complex to exchange with the interlayer cations and (7) calcining the resulting material to decompose the hydroxy metal complex (110). A number of newer methods have been developed to make pillared clays (111—117). 

Y3Fe50i2 [7], These complexes have been obtained at 110-120°C, isolated, and studied in the crystalline form and in an aqueous solution. IR spectra suggest that these are monodentate coordinated to the metal ions. NMR data are in accord with an assumption that hydroxy and carboxylate groups attached to the central carbon atom of the citric acid anion are coordinated to the metal cations forming a stable five-membered ring. However, these important data are obtained at conditions that do not correspond to the conditions of the synthesis of precursors. 

Stepwise stability constants decrease regularly in the normal manner 178), log > log K2 > log K, as may be exemplified by the values for the Ni -maltolate system, viz. 5.5, 4.3, and 2.7, respectively 179). Stability constants (log Kf) for a selection of 3-hydroxy-4-pyranonate and 3-hydroxy-4-pyridinonate complexes of some first-row transition metal 2+ cations are listed in Table IV 10,128,180—184). The values for the 3d transition metal cations conform to the long-established Irving-Williams order... 

Separation is based on the reversible chelate-complex formation between the chiral selector covalently bonded to the chromatographic support, and the chiral solute with transition metal cations. Chelation properties of both the chiral selector and the chiral solute are required. Compounds therefore need to have two polar functional groups in a favorable arrangement to each other, like a )3-amino acids, amino alcohols and a-hydroxy acids, which can form rings membered with central chelating metal ions, like Cu(II), Zn(II), Cyclic... 

Reaction of 175 with Cgg yields a hydroxy-functionalized fullerene that can be further derivatized. This hydroxy-fullerene was coupled with a porphyrine unit via a polyethyleneglycol-Hnker. This linker can be arranged similarly to a crown-ether to complex metal cations. Complexation is used to tune the distance between the porphyrin imit and the Cgg-moiety and thus tune the donor-acceptor properties of this porphyrin-fuUerene hybrid [177]. 

The hydrated metal hydroxy complex in Eq. (1.10) is a soluble species. However, if the pH is sufficiently high, the metal hydroxide, which is relatively insoluble for most metals (apart from the alkali group metals) will precipitate. The pH value at which hydroxide precipitation occurs can be related to the acidity of the cation and is approximately equal to the pK of the cation, where the pK is minus the logarithm of the equilibrium constant of Eq. (1.10). 

In the Ba2+ complex with (145), two anions coordinate to the cation in different ways (Figure 32b). The metal ion sits primarily in a cavity provided by one of the anions and is six-coordinated by two ether, two hydroxy, one keto and one carboxylate oxygen atoms. A nine-fold coordination is completed by further coordination to two oxygen atoms from the second anion and a water molecule. 73 A review of the structures of polyether antibiotic complexes is available and includes a compilation of structural data.372 The stoichiometries of alkali and alkaline earth complexes of (145) in methanol, have been determined potentiometri-cally and show 1 1 neutral complexes for the alkali metal cations, and high stability 1 1 (charged) and 1 2 (neutral) complexes for the alkaline earth cations.574... 

Figure 5. Stahility constants of hi hydroxy complexes of divalent metal cations of the first transition series. Wagman et al. (36) (X) Baes and Mesmer (23) ( JYatsimirskii and VasiVev (11) (A) Smith and Martell (16).

Chiral di mination studies with the tetra-amine 54 were also performed with respect to racemic molecular anions via the formation of cascade complexes. Regulation of the chiral discrimination may be achieved to some extent by the nature of the cation initially complexed. Efficient ion pairing with molecular anions is favoured in solvents of low polarity. In this respect, aqueous solutions of alkali metal salts of ( )-mandelic acid or (+)-a-hydroxy-l-naphthaleneacetic acid were extracted into a CDCI3 phase where the ligand 54 is dissolved. In case of the mandelate anion, the anion ligand ratios in the CDCI3 layer were as follows Na (0.6 1), (1 1),... 

Metal ions in aqueous solution generally hydrolyze to form a series of mononuclear ind polynuclear hydroxy complexes. Iron(III) for example forms [FeOH] ", [Fe(OH)2], Fe(OH)3(aq), [Fe(OH)4] , [Fe2(OH)2] and probably other polynuclear species. The formation af a monohydroxy complex (equation 1) can be used as a guide to the degree of electron withdrawal by the cation. Typical pJCa values for various cations are listed in Table 2. (The pJCa values of and HjO are -1.7 and 15.7 respectively.) The tetrahedrally coordinated [Be(H20)4] ion is more acidic than expected when compared with octahedrally coordinated aqua ions of divalent metals. Each hydrogen must carry a somewhat greater portion of the positive charge than it would in an octahedral complex, and this makes removal of a proton easier. Similar considerations apply to square planar Pd (aq). Mercury(II) and tin(II) are also more acidic than would be expected. The high acidity of mercury(II) is due to the unusual stability of linear H2O—Hg—OH, while Sn (aq) has between two and three coordinated water molecules in the inner coordination sphere. ... 

In simple adsorption from aqueous solution, Hg has features in contrast and in common with the base metals. The hydroxy-cation is the active species in the model for heavy-metal adsorption and this also appears to be true for Hg. However, in contrast with Cu, Pb and Zn, the adsorption is less efficient and is strongly inhibited by the formation of halide complexes, as has been shown by Forbes et al. (1974) (Fig. 12-1). These authors also demonstrate that the adsorption of Hg to goethite is effective at pH as low as 4, allowing it to be trapped subsequent to sulphide oxidation. Whilst many minerals in weathered rocks and soils may each adsorb Hg, the relative efficiency of the hydrous iron oxides (Andersson, 1979) implies that these phases will be the dominant host in most exploration samples. However, the soil organic matter is also of importance and, although the association with Hg has been described as adsorption, it seems more... 

The metallic cation in solution exists as aqua complex ions in equilibrium with their respective hydroxy complex ... 

Accurate modeling of his Th adsorption data as a function of pH required Riese (1982) to assume that the metal-hydroxy complexes were strongly adsorbed. Other researchers have often found this same assumption necessary when TL modeling the adsorption of strongly OH-complexed cations. Tables 10.12 and 10.13 list A " values for a number of metal-hydroxy complexes. 

This term is, unfortunately, used to describe several quite different chemical reactions. Hydrolysis as described here should not be confused with the reactions of metal cations with water to form hydroxy-metal complexes. 

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