Second coordination shell

A series of model siderophore molecular recognition studies coupled with host-guest carrier facilitated model membrane transport studies was reported (198-202). Three approaches were taken which incorporate (i) second coordination shell host-guest complexation, (ii) ternary complex formation, and (iii) a combination of ternary complex - second coordination shell host-guest complex formation. Examples of these approaches are described below. 

Second coordination shell complexation of hydrophilic ferrioxamine B produces a hydrophobic supramolecular siderophore-ionophore assembly. The hydrophobic characteristic in concert... 

A combination first coordination shell-second coordination shell based recognition BLM transport system was devised, including active transport (200). This is based on a labile dihydroxamic acid system, including alcaligin, and a free lysine hydroxamic acid ligand capable of ternary complex formation to... 

In order to obtain data with reduced temperature smearing, experiments were also carried out at 77 K. However, such experiments could not be carried out in. situ and the catalysts were thus exposed to air before the measurements. EXAFS data of three catalysts with Co/Mo atomic ratios of 0.0., 0.25, and 0.50 were obtained. The results show many similarities with the data recorded in situ and were fitted in a similar fashion using phase and amplitude functions of the well-crystallized model compound M0S2 recorded at 77 K. The results, which are given in Table III, show that the bond lengths for the first and second coordination shell are the same for all the catalysts and identical to the values obtained for the catalyst recorded in situ (Table II). The coordination numbers for both shells appear, however, to be somewhat smaller. Although coordination numbers determined by EXAFS cannot be expected to be determined with an accuracy better than + 20, the observed reduction... 

Two coordination shells were found for Co in SAN, one at 2.33 and the other at 3.50 A (phase corrected). These two peaks correspond to Co-Si direct bonds (first coordination shell) and Co-Co scattering (second coordination shell). Only one major... 

EXAFS data at the Co and Al K edges for Co Al - Cl LDHs were consistent with an ordered array of cations for n = 2,3 and 4. In each case the Al has a second coordination shell of 6 Co ions and the Co has a second shell of 6/n Al and (6-6/ ) Co next nearest neighbors [46]. The peaks arising from the focusing effect are still observed in the Co K edge EXAFS of the material showing that the cobalt cations in the layers remain ahgned. 

The simplest reaction on a metal ion in aqueous solution is the exchange of a water molecule between the first and second coordination shells. This reaction is fundamental in understanding not only the reactivity of metal ions in chemical and hiological systems hut also the metal-water interaction. The replacement of a water molecule from the first coordination shell represents an important step in complex-formation reactions of metal cations and in many redox processes (1). 

Water exchange on cationic lanthanide chelates can also be influenced by the nature of the counter-anions (170,171). Anions like halides, sulfate, nitrate, acetate, and fluoroacetate impose different order on the second coordination shell around the chelate by influencing the hydrogen bond network. Anions with a high charge density like CU and S04 can break up the hydrogen bond network between water molecules around the metal center and by that, slow down the water exchange rate of the inner shell water molecule (171). 

Figure A. Solvent assisted dissociation, or SAD mechanism X-solvent molecule or other ligand in second coordination shell.

Blenzen, A., Foglia, F., Furet, E., Helm, L., Merbach, A. E., and Weber, J. (1997). Second coordination shell water exchange rate mechanism Experiments and modelling on hexaaquochromium(III). J. Amer. Chem. Soc. 118, 12777-87. 

The average lifetime of water molecules in the second coordination shell can be obtained from the MD simulation and was found to be in the range of 20-25 ps for the poly(amino-carboxylate) complexes and 56 ps for the polyamino-phosphonate based [Gd(DOTP)]5- [97]. These residence times are long if compared to those of second sphere water around the aqua ions of lan-... 

The EXAFS spectroscopy results strongly confirm the existence of local order in the mineral part of lead isooctane reverse micelles in dodecane and reveal quantitative information concerning the first and second coordination shells. The radial distribution functions (RDFs) exhibit peaks at around 0.19 nm, corresponding to the first shell of oxygen atoms and at around 0.35 nm corresponding to the shell of lead. Analytical transmission electron microscopy (ATEM) indicates the size of the mineral core of the micelles (1-1.5 nm) and the discoid shape of the particles when the micelles aggregate (Mansot et al. 1994). 

The complexes with n=5 and n=6 exhibit marked differences from the corresponding K+ systems with the fifth and sixth molecules located in the second coordination shell forming bifurcated H-bonds with the first solvation sphere (as shown in Figs. 7 and 8). 

Fig. 4.8. Change in bond strength with a surface atom by a coadsorbate in the second coordination shell with respect to adsorbate (schematic), w = bond weakening, s = bond lengthening.

The effect on the bond strength of a coadsorbate adatom B, adsorbed in the second coordination shell with respect to adatom A, is an increase of the bond energy (see Fig. 4.8). The weakened interaction between metal atoms 1 and 2, results in an increase of the M-B bond strength. 

Fig. 4.62. The second coordination shell tetrahedra with respect to a proton in a zeolite.

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