Water cation complexes

The complexes on surface of chelate-functionalized silica often include ligands available in solution in the coordination sphere. Use of a chromophore reagent as a ligand leads to the formation of colored mixed ligand complexes (MLC). The phenomena can be used as a basis for developing test-systems for visual determination of microquantities of inorganic cations in water. 

Following the transition state 6TS2 downhill, we end up with the cationic iron water complex 63, which is found -70 or -67 kcal/mol more stable than the reactants at the CCSD(T) or CASPT2 levels, respectively. The B3LYP numbers are similar, yielding -73... 

Fig. 27 Competition by water and sulfide for cation complexed persulfoxide and its effect on the sulfoxide/sulfone ratio as a function of zeolite loading.

A ferris wheel assembly involving a 1 1 complex of 19 and metallated [18]crown-6 is found in the cationic supermolecule [La(H20)3([ 18]crown-6)] (19+2H) + [48]. The lanthanum ion is coordinated by one calixarene sulfonate group, the [18] crown-6 and three aquo ligands, and the metallated crown sits inside the calixarene cavity. A helical hydrogen bonded chain structure is formed between the cationic assembly, water and chloride ions. The ferris wheel structural motif is also found in Ce3+ complex which simultaneously contains a Russian Doll assembly [44]. 

Some caution is required when comparing the association constants obtained from extraction experiments with those measured under anhydrous, homogeneous conditions. Iwachido et al. (1976, 1977) have shown that the extracted cation retains part of its aqueous solvation shell on complexation. In particular, the small univalent cations (Li+, Na+) and bivalent cations give high hydration numbers for their crown-ether complexes. Water molecules completing the co-ordination sphere of the cation have frequently been encountered in the solid state of crown-ether complexes (Bush and Truter, 1970, 1971). The effect of small amounts of water on the equilibria (1) has not been studied yet for crown ethers. However, it has been found that the presence... 

Rate and equilibrium constants for metal-cation complexes of 15-crown-5 and 18-crown-6 in water at 25 °C... 

Whereas Gold and Sghibartz showed that cation complexation depressed the rate of crown-ether disrupture, there is convincing evidence that crown ether formation is facilitated by the presence of cations. The template effect, presumably due to complexation of the open-chain precursor and formation of a crown-type conformation, clearly emerged from studies in which the yield of crown ethers was related to the type of cations present (Reinhoudt et al., 1976). Kinetic evidence for the template effect was presented by Mandolini and Masci (1977), who showed that the rate of cyclization of the precursor of benzo-18-crown-6 [2061 decreased in the order Ba2+ > SrJ+ > K+ > Na+ > Li+. This sequence is the same as the one found for the stability constants of the 1 1 complexes of these cations with 18-crown-6 in water (Table 3). 

When a salt is dissolved in water, the metal ions, especially transition metal ions, form a complex ion with water molecules and/or other species. A complex ion is composed of a metal ion bonded to two or more molecules or ions called ligands. These are Lewis acid-base reactions. For example, suppose Cr(N03)3 is dissolved in water. The Cr3+ cation attracts water molecules to form the complex ion Cr(H20)63+. In this complex ion, water acts as the ligand. If ammonia is added to this solution, the ammonia can displace the water molecules from the complex ... 

Fraser and Taube (1959), in their pioneer work, studied the interaction of Cr + or ions with complex salts containing a cation of methylmaleatopentamminocobalt. In acid medium (HCIO4 + H2O), a redox reaction takes place. As a result of this reaction, the positive charge in the complex cation decreases by unity and the complex cation exchanges water for the methylmaleate ligand. 

It is noteworthy to mention that the two water molecules in [Ho(HNIC)3 (0112)2] " cationic complex are occupying trans-positions across the diagonal of a square face (Fig. 21) while they occupy cis (vicinal positions in [M(NIC)3(OH2)2] dimers and in diketonate complexes described earlier. 

B) The AG -values derived from formation constants of complexes in methanol (cf. Table 2) are not identical with the free energies of transfer AG, as defined by Eq. (30), because the cations are already solvated by the nonaqueous solvent prior to the complexation. These values are representative, none the less, since the solvation of cations in water and methanol is very similar (95). 

Figure 10 (a) Relation between log K, and the ratio of cation diameter to cavity size for dicyclohexyl[18]crown-6 in water at 25 °C (b) logJST, values of several crown ether-alkali cation complexes... 

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