Anionic interactions oxygen donors

A route for designing Gd(HI) complexes whose relaxivity depends on the presence of lactate, is provided by the ability shown by some hexa- or hepta-coordinate chelates to form ternary complexes with a wide array of anionic species (154-161). The interaction between the coordinatively unsatured metal complex and lactate involves the displacement of two water molecules coordinated to Gd(III) ion with the two donor atoms of the substrate, thus leading to a marked decrease in the relaxivity. Lactate is a good ligand for Gd(IH) ion because it can form a stable 5-membered ring by using the hydroxo and carboxylic oxygen donor atoms (Fig. 19). 

A study of the interaction of ligand (88) with alkali metal complexes has been reported.423 The NaNCS complex contains the ligand bound to the Na+ by five oxygen donors but the sulfur does hot interact at all the six coordination of the cation is completed by the NCS- anion.424 The KNCS complex has the metal at the centre of a hexagon of donors, all of interact with the metal, 2S and in the RbNCS complex the Rb+ sits out of the plane towards an N-bonded NCS- anion and is also coordinated by a sulfur from an adjacent ring to give a dimeric structure.426... 

Neutral extracting agents possessing oxygen-donor atoms (hard bases) in their structure easily coordinate trivalent lanthanide and actinide cations, but do not discriminate between the two families of elements, because the ion-dipole (or ion-induced dipole type) interactions mostly rely on the charge densities of the electron donor and acceptor atoms. As a result, the similar cation radii of some An(III) and Ln(III) and the constriction of the cation radius along the two series of /elements make An(III)/Ln(III) separation essentially impossible from nitric acid media. They can be separated, however, if soft-donor anions, such as thiocyanates, SCN-, are introduced in the feed (34, 35, 39, 77). 

The cr-radical (III) and 02 are considered to be formed by electron transfer from the skatole anion to oxygen in the ternary complex (II), which is composed of a strong electron donor and a weak electron acceptor through Fe(II)-porphyrin (see Reaction 4). In such a complex (D-Fe(II)P-A type complex, D = electron donor, A = electron acceptor), electron transfer from the donor to the acceptor should occur more easily than the direct electron transfer in D-A complex, because in the former the cooperative interaction of the three components should decrease the energy barrier of the electron transfer. 

Further, the kinetics of Ca binding are quite different with water exchange rates close to the coUision diffusion limits of 10 ° s unlike much slower rates of 10 s for Mg. Ca can interact with neutral oxygen donors, like carbonyls and ethers, unlike Mg in aqueous media, as well as with anions, which avoids compehhon with Na. ... 

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