Replacement of a water molecule

Nien either by hydrogen bonding interaction or by replacement of a water molecule (Figure 9). General acid catalysis is not... 

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). 

Nitric oxide forms a brown ring in cold ferrous sulfate solution (brown ring test for nitrates). Tbe reaction involves the replacement of a water molecule by nitric oxide in bydrated ferrous ion ... 

A similar situation holds for the reaction of OH with Cu2+. The reaction proceeds by the replacement of a water molecule of its solvation shell (Cohen et al. 1990) rather than by ET. In neutral solution, the intermediate formed carries zero charge [reaction (37) Barker and Fowles 1970 Asmus et al. 1978 Ulanski and von Sonntag 2000], and only in more acid solutions more positively charged species start to dominate (Fig. 3.1), but real aqua-Cu3+ may not to be formed to any major extent, because at below pH 3 the reaction becomes increasingly reversible [reaction (40) Meyerstein 1971 Ulanski and von Sonntag 2000]. 

Dispersion forces give rise to an interaction energy in which the potential energy of interaction varies as r , where r is the distance between the centers of the two substances interacting. Thus, the equation for the dispersive energy of interaction may be written as A/r , where A is a constant independent of r. The rapid decrease of such forces with increase of distance from the origin makes it unnecessary to consider dispersion interactions outside the primary solvation shell by then, they have already decreased to an extent that they no longer warrant consideration. Inside the primary hydration sheath, the dispersion interaction can be treated in the same way as the ion-dipole interaction. That is, in the replacement of a water molecule by a nonelectrolyte molecule, one must take into account not only the difference in ion-dipole... 

The catalytic cycle starts with the replacement of a water molecule at the axial position by superoxide (Fig. 12). 

Kinetics of Selenium Adsorption. Zhang and Sparks 4G) examined selenate and selenite adsorption and desorption on goethite using pressure jump relaxation techniques. Selenate produced a single relaxation, that was interpreted as outer-sphere complexation with surface protonation based on fitting to the triple layer model. The forward rate constant was 10 L mol s Selenite adsorption was proposed to occur via two steps, an initial outer-sphere complex and subsequent replacement of a water molecule by formation of inner-sphere complexes of both HSeOj and SeOj, based on optimized fits using the triple layer model. The model optimized fit for the pK, of the surface species was approximately 8.7. Forward rate constants for the first step were on the order of 10 L -mor -s for HSeOj and 10 L -mor -s for SeOj. Forward rate constants for the formation of the inner-sphere complexes were 100 and 13 s respectively for HSeOj and SeOj. Agreement between the equilibrium constant obtained from batch and kinetic studies was taken as confirmation of the proposed reactions. 

Kinetics of Molybdenum Adsoiption. Zhang and Sparks 41) examined molybdate adsorption on goethite using pressure jump relaxation experiments. Molybdate adsorption was proposed to occur via two steps, an initial outer-sphere complex and subsequent replacement of a water molecule by formation of an inner-sphere complex of Mo04, based on optimized fits using the triple layer model. Forward rate constants were on the order of 4x10 L mol s and 40 s for the first and second reaction steps. 

Irving and Edgington found that 1 or 2 nitrates could enter into the complex, depending on the nitric acid concentration. With tributyl phosphine oxide (TBPO) the species identified were M(IV) Tg(NOg)P, M(IV) T2(NOg>2 Pg, and M(ni) Tg (NOg)Pg, whereas with P = TBP they were M(in) TgPg, M(IV) Tg(NOg)P, and M(VI) TgP, where M is an actinide. Thus the complex is influenced by the basic strength of P. These authors postulate that the reaction mechanism for hexavalent species is the replacement of a water molecule in the complex by P, giving a coordinately unsaturated product, thus... 

Several successive substimtions by hydroxide ions may occur. This definition of hydrolysis permits us to consider it as a true complexation phenomenon. Reaction (25.1) is a true reaction of a complex formation since it involves the reaction of the hydroxo ligand. Indeed, it consists of the replacement of a water molecule from an initial aqua complex by the hydroxo ligand. 

Chromium(in) complexes also undergo photoanation reactions. When aqueous solutions of Cr(H20)6 are photolyzed in the wavelength range of 40Q-575 nm in the presence of Cl" or SCN" ion, replacement of a water molecule by the anionic ligand occurs ... 

Replacement of each water molecule by an MnAli2 icosahedron with shared faces leads to an infinite framework with 136 Mn and 816 Al atoms in the unit cube. This framework is similar to the framework of covalently bonded carbon atoms in a diamond crystal, with one body diagonal of each pentagonal dodecahedron in place of each C—C covalent bond. 

It was clear for some time that a number of zinc enzymes required two or more metal ions for full activity, but in the absence of X-ray structural data the location of these metal centres with regard to one another was often uncertain. When the first 3-D structures began to appear, it became clear that the metals were in close proximity. A particular feature of many of these enzymes was the presence of a bridging ligand between two of the metal sites, usually an Asp residue of the protein, which is occasionally replaced by a water molecule. While some of the sites contain only Zn ions, several contain Zn in combination with Cu (in cytosolic superoxide dismutases) Fe (in purple acid phosphatases) or Mg (in alkaline phosphatase and the aminopeptidase of lens). 

The first step in the formation of a 1 1 chromium complex by the interaction of an 0,0-dihy-droxydiarylazo compound and the hexaaquachromium ion at low pH must involve replacement of a coordinated water molecule in the hexaaquachromium(III) ion by one of the donor atoms in the azo compound, The next step in the reaction sequence is formation of a chelate complex by replacement of a further molecule of coordinated water. Polarization of the ligand as a result of chelate formation markedly enhances its acidity and proton loss ensues (cf. Section 58.2.2.1). 

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