EXAFS water-metal complexes

The first experimental information on the kinetic parameters for water exchange on a tetravalent metal ion was published in 2000 for U4+ and Th4+ (265,268,271). The coordination numbers for these two complexes were determined by EXAFS to be 10 1. Based on the high coordination number (there are no complexes known with unidentate ligands and coordination numbers larger than 10) a limiting associative mechanism (A) is unlikely and a d-activated mechanism is probable. Surprisingly,... 

As noted earlier, metal ions in polar solvents will form complexes with the solvent molecules. X-Ray diffraction, EXAFS, and visible absorption spectroscopy show that nickel(II) ion in dilute aqueous solution is present as the green hexaaqua complex Ni(H20)62+, just as in solids such as NiS04-7H20, which is actually [Ni(H20)e]S04-H20. In the crystal, the extra water molecule is loosely associated with the sulfate ion independently of the nickel-aqua complex it is sometimes referred to as lattice water, as distinct from complexed water. 

The highest oxidation states for Ir are VI and V, stabilized by ligands such as F. IrFg (a yellow crystalline sohd, /Xeff = 2.9 /xb at 300 K) is formed by the direct fluorination of Ir metal. Molecules of IrFe are octahedral, and the stmcture has been studied in the gas phase by electron diffraction and in the sohd phase by EXAFS. IrFe is hydrolyzed by water, and reactions (1-4) illustrate its general reactivity. Reaction four represents the formation of the first binary, tripositive metal carbonyl complex. ... 

Complex [Am(MPBIZ)2] is the prevailing component in ethanol solution as shown by means of EXAFS experiment [81]. The MPBIZ molecule coordinates to Am through the nitrogen atoms in bidentate mode. The Am-N distance is equal to 2.63 A, while the Am-0 bonds are shorter. By this means, the MPBIZ ligand appears to be bonded to metal atom weaker than water molecules. The authors made the conclusion that the contribution of the covalent bond to the chemical bonds between Am and N is small. 

With his model, Sverjensky (2001) predicted different distances for the adsorption of different electrolyte cations (i.e., Rb+ = 3.3 A, Sr2+ = 2.9 A) at the rutile-water interface that compared well to the distances reported from x-ray standing-wave experiments (Fenter et al., 2000). The model also suggested that trace amounts of metals (e g., Sr"" ", Ca +j other than the electrolyte cations should form inner-sphere complexes if adsorbed to the p-plane of rutile and similar sohds, and form outer-sphere complexes if adsorbed to the p-plane of quartz, goethite, and similar solids. These predictions were consistent with the results of x-ray standmg-wave and EXAFS studies (Axe et al., 1998 Fenter et al., 2000 O Day et al., 2000 Saliai et al., 2000). 

The above sequence has been observed in studies of alkaline earth adsorption on y-Al203 (Huang Stumm, 1973). The trend is also consistent with the expectation based on the expected preference of harder Lewis acids for hard Lewis bases like surface hydroxyls. Limited spectroscopic evidence is available for sorption of alkaline earth metals because many of these metals do not exhibit sufficiently high K-shell fluorescence energies to be studied in the presence of corundum and water using current EXAFS methods. Chen and Hayes (1999) have shown that Sr(II) sorbs to montmorillonite, illite, and hectorite primarily as a weakly associated outer-sphere complex. Similar findings have been reported for sorption of Sr(II) to clay minerals (Parkman et al., 1998 O Day et al., 2000 Sahai et ah, 2000). 

Chisholm-Brause, CJ, Morris DE, Richard RE (1992b) Speciation of uranium(VI) sorption complexes on montmorillonite. Water-Rock Interactions, Proc 7th Int Symp 1 137-140 Chisholm-Brause CJ, O Day PA, Brown GE Jr, Parks GA (1990a) Evidence for multinuclear metal-ion complexes at solid/water interfaces from X-ray absorption spectroscopy. Nature 348 528-531 Chisholm-Brause CJ, Roe AL, Hayes KF, Brown GE Jr, Parks GA, Leckie JO (1989b) XANES and EXAFS study of aqueous Pb(II) adsorbed on oxide surfaces. Physica 158,674-676 Chowdhury TR, Basu GK, Mandal BK, Biswas BK, Samanta G, Chowdhury UK, Chanda CR, Lodh D, Roy SL, Saha , Roy S, Kabir S, Quamruzzaman Q, Chakraborti D (1999) Arsenic poisoning in the Ganges delta. Nature 401 545-546... 

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