Water co-ordinated

In the mechanism of an interfacial catalysis, the structure and reactivity of the interfacial complex is very important, as well as those of the ligand itself. Recently, a powerful technique to measure the dynamic property of the interfacial complex was developed time resolved total reflection fluorometry. This technique was applied for the detection of the interfacial complex of Eu(lII), which was formed at the evanescent region of the interface when bathophenanthroline sulfate (bps) was added to the Eu(lII) with 2-thenoyl-trifuluoroacetone (Htta) extraction system [11]. The experimental observation of the double component luminescence decay profile showed the presence of dinuclear complex at the interface as illustrated in Scheme 5. The lifetime (31 /as) of the dinuclear complex was much shorter than the lifetime (98 /as) for an aqua-Eu(III) ion which has nine co-ordinating water molecules, because of a charge transfer deactivation. 

Figure 2-15. Co-ordinated water ligands show deprotonation processes corresponding to the first and second ionisation steps.

At first sight these reactions are simple examples of metal-activated nucleophilic attack upon the nitrile carbon atom. However, the geometry of the co-ordinated chelating ligand is such that the nitrile nitrogen atom is not co-ordinated to the metal ion (4.3 and 4.4) It was initially thought that this provided evidence for a mechanism involving intramolecular attack by co-ordinated water or hydroxide (Fig. 4-10). However, detailed mechanistic studies of the pH dependence of the reaction have demonstrated that the attack is by external non-co-ordinated water (or hydroxide) (Fig. 4-11). 

We saw in Chapter 2 that co-ordination of a water molecule to a metal ion modifies the pK and can make the water considerably more acidic. This stabilisation of the hydroxide anion is rationalised in terms of transfer of charge from the oxygen to the metal in the coordinate bond. Some typical pKa values of co-ordinated water molecules are given in Table 5-2. 

Using electron spin echo modulation spectroscopy, Narayana et al.33 showed that in 0.25% V205 on Si02 a vanadyl species with two co-ordinated water molecules, giving octahedral co-ordination, is present. This species loses the two water molecules even at 100°C leaving a tetrahedral species. 

The condensation of o-aminobenzamide and pyridine-2-carbaldehyde produced the terdentate ligand (129) which partially isomerizes in methanol, in the presence of nickel(n) nitrate, to form the cyclic ligand (130). The nickel complex [Ni( 129)(130)-(H20)](N03)2,4H20 has been X-rayed, and the results show that (129) bonds as a terdentate N20 donor, and (130) as a bidentate N2 ligand, the co-ordinated water molecule completing the irregular octahedron about the nickel.575... 

Trimeric Cu3(dmg)2X4,nH20 (dmgH = dimethylglyoxime) (163), contain coordinated halide when X = Cl, but co-ordinated water when X = Br. 

Oxygen Donors. The formation and stability constants oi complexes between Pt (Pd, Rh, Ir, Os, Ru) and o-coumaric acid have been determined by pH titration.31 The results indicate that a 1 2 complex is formed with Pt. Acid-base properties of aquo-complexes formed from [Pt(X)2(OH)2(NH3)(MeNH2)] (X = Cl, Br or N02) in aqueous solutions have been examined using potentiometric titration experiments.186 The Ka of co-ordinated water was lower for (X)2 = (H20)2 than for (X)2 = (H20) (OH-). 

Outer and Inner Sphere Complexes. Outer sphere complexation involves interactions between metal ions and other solute species in which the co-ordinated water of the metal ion and/or the other solute species are retained. For example, the initial step in the formation of ion pairs, where ions of opposite charge approach within a critical distance and are then held together by coulombic attractive forces, is described as outer sphere complex formation. 

Note. In acid solutions, hydroxocobalamin takes up a hydrogen ion which converts the hydroxyl group to a co-ordinated water molecule, and in this form it is known as aquocobalamin, which is basic and forms salts with acids. In solution, aquocobalamin exists in equilibrium with hydroxocobalamin and, since it is more stable in acid solution, it usually occurs commercially in the form of aquocobalamin (E. L. Smith et ah. Analyst, Lond., 1962,57, 183- 186). 

Salt hydrates hold water molecules as (i) co-ordinated water, (ii) anion water, (iii) lattice water, or (iv) zeohte water. The term water of constitution is a misnomer often applied to compounds wrongly formulated as FegOg.HgO and MgO.HgO. These are the true hydroxides FeO(OH) and Mg(OH)2 and most of the class have a layer lattice consisting of sheets of OH and Onions with cations between them. 

Co-ordinated water. Many cations form complex ions with four or six molecules of w ater co-ordinated to the metal ... 

The sulphates do not form alums. Small ions such as Al r = 0.50 A) can be surrounded octahedrally by six HgO molecules, but the lanthanide ions (r > 0.85 A) are too large to co-ordinate water molecules in this way. 

The factor which may explain the aptitude of the high charge-density ions, such as Mg " and Li", for the super-equivalent adsorption at Ti02 is their ability to interact sufficiently strongly with the co-ordinating water molecule to weaken the O-H bonds. In other words, a cation of small radius, such as Li, is expected to polarize a water molecule... 

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