Replacement of water molecules

Here the ligand (L) can be either a neutral molecule or a charged ion, and successive replacement of water molecules by other ligand groups can occur until the complex ML, is formed n is the coordination number of the metal ion and represents the maximum number of monodentate ligands that can be bound to it. 

A further factor which must also be taken into consideration from the point of view of the analytical applications of complexes and of complex-formation reactions is the rate of reaction to be analytically useful it is usually required that the reaction be rapid. An important classification of complexes is based upon the rate at which they undergo substitution reactions, and leads to the two groups of labile and inert complexes. The term labile complex is applied to those cases where nucleophilic substitution is complete within the time required for mixing the reagents. Thus, for example, when excess of aqueous ammonia is added to an aqueous solution of copper(II) sulphate, the change in colour from pale to deep blue is instantaneous the rapid replacement of water molecules by ammonia indicates that the Cu(II) ion forms kinetically labile complexes. The term inert is applied to those complexes which undergo slow substitution reactions, i.e. reactions with half-times of the order of hours or even days at room temperature. Thus the Cr(III) ion forms kinetically inert complexes, so that the replacement of water molecules coordinated to Cr(III) by other ligands is a very slow process at room temperature. 

However as well as electrostatics the making and breaking of bonds must be considered. The Be-0 bond has considerable covalent character so that it is not obvious, a priori, that replacement of water molecules by bridging hydroxide should be favored by a negative enthalpy... 

Now, this description of the stepwise replacement of water molecules by metal ions as nearest neighbors to an ion can be linked up with the charge-transfer reaction. To what site is the ion likely to cross the interface ... 

A number of the intensely colored hydrated salts of trivalent chromium (the nitrate, sulfate, chloride, and the alum ) are doubtless familiar. The octahedral hexaaquochromium(III) ion, Cr(H20) 3, is violet, but aqueous solutions of chromic salts are often green as a result of replacement of water molecules in the complex by the anions present. The change of color occurring on heating a solution containing trivalent chromium and chloride ions should be recalled ... 

From a surfactant point of view, one may determine a packing criterion originating from minimising the sum of the two surface contributions to the chemical potential. The surfactant can be divided into a hydrophobic and a hydrophilic part, which are clearly separated. The bulk contribution to the chemical potential due to the replacement of water molecules around hydrocarbon chains by its own hydrocarbon chains is the driving force for assembly. The packing criterion and the consequent optimal number of surfactants in a micelle is determined by the minimization of the surface contribution at a finite N. The first step in testing the model of surfactant assembly is testing the size of spherical micelles as a function of experimental parameters like salt concentration and temperature. 

Solvating extractants compete with water for a position in the first solvation shell of the metal ion. The replacement of water molecules by these reagents facUitates the transfer of the metal-ion complex into an organic phase. 

In two other equilibria (schemes (19) and (21)), replacement of water molecules in the [U02(H20)6] complexes by acid ligands is accompanied by the decrease... 

The process of molecular adsorption at the polarizable interface involves replacement of water molecules in the inner layer which are solvating the electrode. Thus,... 

While the rate of dissociation of Fe(phen)i" is not sensibly affected by hydrogen ions, hydroxyl (200) as well as cyanide and azide ions 201) greatly accelerate the rate of displacement of the diimine ligand. For the reaction with OH, the activation energy for the catalized path is lowered by ca. 6 kcal/mole 202). The preferred mechanism 201) consists in the replacement of water molecules in one of the pockets between the hgands, by the nucleophilic ion. Interaction between the i r-orbitals of the latter and the antibonding metal i-orbitals is believed to destabilize the metal-diimine bonds. 

Coming back to the decarboxylation of 6-NBIC, cholesterol was found to reduce kves for DHAB vesicles by a factor of 3 when at 50 mol % in the bilayer. As a result of its appreciable hydrophobic surface area, it penetrates significantly into the bilayer, thereby decreasing the inter-amphiphile interactions. Under these conditions, the hydration of the interface is increased, and the reactant is stabilized. On the other hand, for trehalose as the additive, the value of kves is slightly increased. This is in accord with the notion that binding to the bilayer surface leads to replacement of water molecules from the vesicular interface with a concomitant destabilization of bound 6-NBIC. 

Step 2. Hydrated silicate species interact with hydrated organic cations, resulting in replacement of water molecules in the coordination sphere of the TPA by silicate anions. The hydrophobicity/hydrophilicity of the organic is known to be critical in this stage. (Replacement of TPA with ethanoltripropylammo-nium ions results in much slower crystallisation of the pure silica ZSM-5, whereas diethanoldipropylammonium does not nucleate the structure.)... 

An example of solvating extraction with solvating agents is shown in Equation (18.4). The chemistry is in the replacement of water molecules by the ion exchanger according to the possible... 

The stepwise replacement of water molecules by dimethyl sulphoxide molecules in [Cr(0H2)6][C104]3 has been shown to take place with intermediates of general formula [Cr(OH2)n(BMSO)6-n] " - The stepwise formation constants were found to be similar in magnitude. ... 

As metal ions in aqueous solutions form hydrated aqua complexes, complex-formation processes should be considered as reactions involving replacement of water molecules by ligand particles. Kinetic data for the formation of metal complexes obtained by various methods showed [27] that the formation of ML complexes proceeds via a rapid pre-equilibrium, resulting in the formation of an outer sphere complex M(H20)L. Then, this intermediate loses water in the rate-determining step of inner sphere complex (ML) formation. 

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