Water molecule, coordination

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. 

The mode of extraction in these oxonium systems may be illustrated by considering the ether extraction of iron(III) from strong hydrochloric acid solution. In the aqueous phase chloride ions replace the water molecules coordinated to the Fe3+ ion, yielding the tetrahedral FeCl ion. It is recognised that the hydrated hydronium ion, H30 + (H20)3 or HgO,, normally pairs with the complex halo-anions, but in the presence of the organic solvent, solvent molecules enter the aqueous phase and compete with water for positions in the solvation shell of the proton. On this basis the primary species extracted into the ether (R20) phase is considered to be [H30(R20)3, FeCl ] although aggregation of this species may occur in solvents of low dielectric constant. 

Fig. 12 (A) The d(CGCGAATTCGCG)2 duplex with a narrow groove and a sodium ion coordinated at the ApT step. (I) The DNA is shown in stick representation and the ion in space-filling size. Left view is directly into the central minor groove. Right view left view rotated 90° counterclockwise and tilted 30° to show the ion in the minor groove. (II) The base pair views are of the central ApT step. Top view is down the helix axis, bottom view is directly into the minor groove. (B) The DNA duplex with a phosphate-oxygen pair-sodium ion interaction and a water molecule coordinated at the ApT step. (II) Views as in Fig. 12A for the phosphate-ion-water-base complex at the AT site. Reproduced with permission from Ref. (42). Copyright 2000, American Chemical Society.

In the spectra of alkyl cobinamides two peaks have been observed at 3.89 and 4.42 which were assigned to the protons of a water molecule coordinated at the lower axial site (130). To confirm this assignment, it was found that addition of cyanide to methyl cobinamide, which displaces coordinated water, caused the peaks to disappear. Likewise, addition of excess D2O caused disappearance of the peaks through either ligand exchange or proton-deuteron exchange. 

The mechanism given is in support of the existence of inner-sphere surface complexes it illustrates that one of the water molecules coordinated to the metal ion has to dissociate in order to form an inner-sphere complex if this H20-loss is slow, then the adsorption, i.e., the binding of the metal ion to the surface ligands, is slow. 

As an example, infrared spectroscopy has shown that the lowest stable hydration state for a Li-hectorite has a structure in which the lithium cation is partially keyed into the ditrigonal hole of the hectorite and has 3 water molecules coordinating the exposed part of the cation in a triangular arrangement (17), as proposed in the model of Mamy (J2.) The water molecules exhibit two kinds of motion a slow rotation of the whole hydration sphere about an axis through the triangle of the water molecules, and a faster rotation of each water molecule about its own C axis ( l8). A similar structure for adsorbed water at low water contents has been observed for Cu-hectorite, Ca-bentonite, and Ca-vermiculite (17). 

As noted in table 11.1, the ability of THFTCA to separate LJO from trivalent lanthanide ions is mainly of enthalpic origin. Reaction 11.33 has a considerably more unfavorable enthalpic contribution than reaction 11.32. The complexation is, however, predominantly entropy driven because the T ArS° term dominates the ArH° contribution for all systems. The large positive entropy changes observed for reactions 11.32 and 11.33 result from the release of water molecules coordinated to the metal on complexation with the tridentate THFTCA2- ligand. Note that a negative entropy contribution would be expected if these reactions were truly 2 particle = 1 particle reactions [226]. 

The carboxyl group (-COOH) of organic acids interacts either directly with the interlayer cation or by forming a hydrogen bond with the water molecules coordinated to the exchangeable cation on the soil-solid and sediment-solid clay... 

Fig. 5-11. A simple model of an interfacial compact double layer on metal electrodes H20,j = adsorbed water molecule H20. = water molecule coordinated with ions H20 . = free water molecule ih = hydrated ions. [From Bockiis-Devanathan-Muller, 1963.]...

In macromolecules, slow exchange effects often quench the relaxivity (Pig. 30) (37) even in the presence of water molecules directly coordinated to iron(III) (91). For instance, in methemoglobin the relaxation rates are attributable to one water molecule coordinated to the paramagnetic center... 

For instance, Lowe et al. showed that the relaxivity of a series of macro-cyclic Gd(III) complexes bearing (3-arylsulfonamide groups is markedly pH-dependent (Fig. 15) on passing from about 8 s mM at pH < 4 to ca. 2.2 s mM at pH > 8 in one selected case (Chart 12, ligand 2) (130). It has been demonstrated that the observed decrease (about 4-fold) of ri is the result of a switch in the number of water molecules coordinated to the Gd(III) ion from 2 (at low pH values) to 0 (at basic pHs). This corresponds to a change in the coordination ability of the p-arylsulfonamide arm that binds the metal ion only when it is in the deprotonated form (Fig. 15). 

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). 

Clay minerals behave like Bronsted acids, donating protons, or as Lewis acids (Sect. 6.3), accepting electron pairs. Catalytic reactions on clay surfaces involve surface Bronsted and Lewis acidity and the hydrolysis of organic molecules, which is affected by the type of clay and the clay-saturating cation involved in the reaction. Dissociation of water molecules coordinated to surface, clay-bound cations contributes to the formation active protons, which is expressed as a Bronsted acidity. This process is affected by the clay hydration status, the polarizing power of the surface bond, and structural cations on mineral colloids (Mortland 1970, 1986). On the other hand, ions such as A1 and Fe, which are exposed at the edge of mineral clay coUoids, induce the formation of Lewis acidity (McBride 1994). 

When a more acidic oxide is needed, amorphous silica-alumina as weU as meso-porous molecular sieves (MCM-41) are the most common choices. According to quantum chemical calculations, the Bronsted acid sites of binary sihca-alumina are bridged hydroxyl groups (=Si-OH-Al) and water molecules coordinated on a trigonal aluminum atom [63]. Si MAS NMR, TPD-NH3 and pyridine adsorption studies indicate that the surface chemistry of MCM-41 strongly resembles that of an amorphous sihca-alumina however, MCM-41 has a very regular structure [64, 65],... 

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