Outer coordination site

Endicott and Taube consider that there is cause for doubt over the generally-held views that Cr(bipy)3 is oxidised by an outer-sphere mechanism"". They suggest that, since the complex is very labile to substitution, coordination sites... 

Numerous dendrimers, which contain coordination sites on the outer surface, within the macrostructure throughout all layers, and at the inner core have been prepared. In contrast,... 

On the other hand, if it were possible to block the coordination sites of a metal by using a strongly attached ligand such as cyanide, then "outer-sphere electron transfer reactions should become more important (12). 

In outer-sphere reactions, electron transfer occurs without the utilization of a bridging ligand. The existence of such pathways is suggested for reactants which have no readily available coordination sites for forming a chemical bridge, as in equation (1), and where at least one of the reactants undergoes slow ligand substitution on the timescale for electron transfer, e.g. reaction (6). 

It is important to realize that for the inner-sphere or atom transfer pathways in reactions (4), (10) or (12) to appear at the expense of accessible outer-sphere pathways, there are heightened mechanistic demands on the reactants which must be met. For the cases cited there is a necessity for the substitutional lability of Cr11 in reaction (4), the existence of an open coordination site in Co CNJs3- in reaction (10), or the availability of the RuIV=0 group to act as an electron-proton acceptor in reaction (12). 

A conditio sine qua non for catalytic activity (with the exception of outer sphere electron transfer reactions) is that the species present in solution should be coordi-natively unsaturated, it must possess a vacant coordination site for attachment of the substrate to the metal center. Hence, the first steps in many catalytic processes are ligand dissociation and ligand (substrate) coordination (Equations 8-14). 

Although the presence of merely thermodynamic, rather than additional intrinsic, inner-sphere catalysis can yield substantial rate enhancements, these will be limited inevitably by the availability of surface coordination sites. Thus, for example, a bulk reactant concentration of ImM and a maximum (i.e. monolayer) surface concentration of 5 x 10 10mol cm-2 yields a maximum rate enhancement of 5 x 104 fold resulting from "thermodynamic catalysis if Sr = 1 A for the alternative outer-sphere pathway. Greater inner-sphere rate accelerations require pathways yielding increases in /ce) or decreases in AG. ... 

Galactose oxidase hinds a single copper ion within Domain 11 on the axis of the wheel. The active site (Fig. 5) is unhke any other biological copper complex, an appropriate distinction for this remarkable enzyme. To explore the site in more detail, the protein environment of the mononuclear copper center may be separated into (A) direcdy coordinated metal hgands (hrst shell, inner sphere interactions) and (B) the extended active site environment (the second shell or outer coordination sphere). 

As is discussed immediately below, the structures of the type 1 sites in hCp, Lac, and AO—and in Fet3p—differ in some details nonetheless, the electron transfer reactions at this site in the four proteins are all outer sphere in nature (Solomon et al., 1996). This mechanism is dictated by the fact that type 1 sites, as a class, do not have exchangeable, solvent-accessible inner coordination sites to which a reductive ligand could... 

Porphyrin derivatives of Tl have been prepared the metal ion lies 0.9 A above the N4 plane, with an anion coordinated as the fifth ligand. A T1 L2 species is formed with 1,4,7-triazacyclononane two tridentate hgands are coordinated to a Tl ion in a distorted octahedron. It is an inert species with no additional labile coordination sites, and can therefore only be reduced via an outer-sphere electron-transfer process. Crown ether complexes are known only for R2TI salts. ... 

Dendrimer-type ligand (32) serves as a lanthanide container to exhibit on-off switchable luminescence upon lanthanide complexation in response to external anions [56]. Because of the presence of two classes of coordination sites for the lanthanide cations at the inner and outer spheres, the dendrimer 32 exhibits two different binding modes to afford on-off lanthanide luminescence, in which outer complexation at the tetradentate tripod site offers the on luminescence state upon quinoline excitation whereas, inner complexation at the multidentate core site corresponds to the off luminescence state. Upon complexation of 32 with Yb(CF3 SO3 )3, the quite weak NIR luminescence from the Yb(III) center suggests that the Yb(III) ion is most probably located at the inner coordination sites and apart from the excited quinoline moieties. Nevertheless, addition of SCN anion to the 32-Yb(CF3803)3 system induced remarkable spectral changes around the quinoline absorption band and about ninefold enhancement in luminescence intensity at around 980 nm. As the intense Yb luminescence appeared upon quinoline excitation, the employed SCN anion promoted the tripod-Yb +... 

This result seemed to support Larsson s hypothesis (27f 29) that the same criterion shall apply to the outer sphere of coordination as to the inner one—i.e., the ligands must be assumed to occupy a limited number of so-called coordination sites which are spatially well-deflned. How-... 

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