Aqueous solution of transition metal ions

Figure 26-2 Absorption spectra of aqueous solutions of transition metal ions.

A Figure 23.4 Aqueous solutions of transition metal ions. Left to right Co, Ni +, Cu +, and Zn. The counterion is nitrate in aii cases. 

Aqueous electrolyte solutions of transition metal ions exhibit a rich variety of behaviors. All are suitable for study by ND with the exception of the monoisotopic cation Mn and Co +, which have been successfully investigated by TX (Table III). 

Because we live and carry out our chemistry in a world in which water is ubiquitous, we tend to forget that transition metal ions in aqueous solutions are present as aqua complexes and should not be regarded as free ions . In aqueous solution many transition metal ions are reduced by L-ascorbic acid. A typical example in which the mechanism of the reaction has been studied is that with cobalt (iii) ions in water. Cobalt (iii) ions have only a comparatively brief existence in water, but exist long enough to allow kinetic experiments to be performed on their reaction with L-ascorbic acid. The high positive charge on the cobalt causes the water molecules in the aqua complex to be acidic, so that hydroxo species are formed at higher pH values. The effect of this on the reduction reaction is reflected in the mechanism shown in equations (30)-(34). 

The color of metal complexes is basically controlled by three kinds of transition charge-transfer, tt tt and n tt" transitions in complexes with organic ligands, d—d transitions within the metal ion. The latter are usually weaker than the former two, nevertheless the color of aqueous solutions of transition metals is caused by... 

The most common and important complex ions are hydrated metal ions. The coordination numbers and structures of some of these simple complexes have been determined. Isotope dilution techniques were used to show that Cr and Al are bonded rather firmly to six water molecules in aqueous solutions. The interpretation of the visible spectra of solutions of transition metal ions using CFT indicates that ions such as Mn, Fe, Co, Ni, Cr, and Fe are octahedral [M(H20)6] species. For non-transition metal ions it has been more difficult to obtain structural information. Flowever, nuclear magnetic resonance spectroscopy demonstrates that Be in aqueous solution is surrounded by four water molecules. These data support the importance of six coordination. The only exception cited here is Be, an element which obeys the octet rule. 

Common examples of Lewis bonding are found in the complex ions formed by the transition metals (see Chapter 13). Charged metal ions become surrounded by water molecules in aqueous solution. Many transition metal ions become surrounded by six water molecules for example Ee (aq), exists as the hexaaquairon(iii) ion [Fe(H20)5] (Figure 18.1). The six water molecules in this complex each donate a lone pair of electrons from the oxygen atoms of their water molecules to the empty 3d orbitals of the central iron(iii) ion. The water molecules, known as ligands, are acting as Lewis bases (electron pair donors) and the iron(ni) ion is acting as a Lewis acid (electron pair acceptor). 

The latter reactions are catalyzed by a number of transition metal ions which can exist in several oxidation states in aqueous solution, e.g.,... 

Organic hydroperoxides have also been used for the oxidation of sulphoxides to sulphones. The reaction in neutral solution occurs at a reasonable rate in the presence of transition metal ion catalysts such as vanadium, molybdenum and titanium - , but does not occur in aqueous media . The usual reaction conditions involve dissolution of the sulphoxide in alcohols, ethers or benzene followed by dropwise addition of the hydroperoxide at temperatures of 50-80 °C. By this method dimethyl sulphoxide and methyl phenyl sulphoxide have been oxidized to the corresponding sulphone in greater than 90% yields . A similar method for the oxidation of sulphoxides has been patented . Unsaturated sulphoxides are oxidized to the sulphone without affecting the carbon-carbon double bonds. A further patent has also been obtained for the reaction of dimethyl sulphoxide with an organic hydroperoxide as shown in equation (19). 

Enthalpies, Entropies, and Gibb s Energies of Transition Metal Ion Oxidation-Reduction Reactions with Hydrogen Peroxide in Aqueous Solution (T = 298 K) [23]... 

The values of the rate constants for the reactions of transition metal ions with hydrogen peroxide in an aqueous solution are presented in Table 10.2. 

Rate Constants of Transition Metal Ion Reactions with Hydrogen Peroxide in Aqueous Solutions... 

Redox potentials of transition metal ions in aqueous solutions... 

Coordinative Environment. The coordinative environment of transition metal ions affects the thermodynamic driving force and reaction rate of ligand substitution and electron transfer reactions. FeIIIoH2+(aq) and hematite (a-Fe203) surface structures are shown in Figure 3 for the sake of comparison. Within the lattice of oxide/hydroxide minerals, the inner coordination spheres of metal centers are fully occupied by a regular array of O3- and/or 0H donor groups. At the mineral surface, however, one or more coordinative positions of each metal center are vacant (15). When oxide surfaces are introduced into aqueous solution, H2O and 0H molecules... 

In aqueous solutions, in which the most probable ligand is the water molecule, most of the lower oxid ation states (i.e. + 2, + 3 and some of the + 4 states) of transition metal ions are best regarded as hexaaqua complex ions, e.g. [Feu(H20)6]2 +. In these ions the six coordinated water molecules are those that constitute the first hydration sphere, and it is normally accepted that such ions would have a secondary hydration sphere of water molecules that would be electrostatically attracted to the positive central ion. The following discussion includes only the aqua cations that do not, at pH = 0, undergo hydrolysis. For example, the iron(III) ion is considered quite correctly as [Fe(H20)6]3 +, but at pH values higher than 1.8 the ion participates in several hydrolysis reactions, which lead to the formation of polymers and the eventual precipitation of the iron(III) as an insoluble compound as the pH value increases, e.g. ... 

In fact, the converse is observed. The main features of the spectra of transition metal ions in solution are very similar to those for crystal lattices where the same donor atom is present as an anion. Further, the spectra differ little between solids provided the nearest-neighbour atom is unchanged, even if it is part of a multi-atom species and even if the symmetry of the crystal structure is low. The spectra of the first transition series carbonates, for example, are not markedly different from those of their oxides, nor from those of the ions in aqueous solution. In each case the nearest-neighbour atom is oxygen and six of these surround the metal atom in approximately octahedral positions. 

The autoxidation of aqueous solutions of sulfur dioxide (sulfite, bisulfite) is a classic problem in chemistry. Basic features of this reaction have been known since early in this century, when it was established that the reaction is trace metal ion catalyzed (1 ) and most likely involves free radicals (2). Certain chemical effects associated with sulfite autoxidation were noted also. Before the turn of the century, it was noted that sulfite would induce the oxidation of transition metal ions (3) and it was reported later that the oxidation of organic compounds was brought about during sulfite autoxidation ( 0. Conversly, organic compounds were also shown to serve as inhibitors of sulfite autoxidation (5). 

Leaching of ions and break-down of silicates During leaching of a mineral an ion is removed from a site in a crystal structure to an aqueous phase. Most transition metal ions are present in six-coordinated sites in silicate minerals and exist in aqueous solutions as hexahydrated ions, [A/(H20)6]"+. The crystal field splittings summarized in table 2.5 indicate that CFSE s of ions in aqueous solutions and oxide structures are comparable,... 

Oxidation of transition metal ions in sedimentary processes Under the relatively oxidizing environments existing near the Earth s surface provided by aerated aqueous solutions in contact with the atmosphere, several... 

Sedimentary geochemistry. The aqueous phase dominates weathering, leaching, transportation, and precipitation processes in the sedimentary cycle. The behaviour of transition metal ions to chemical attack depends on the relative stabilities of hydrated and complex ions in solution and bonded cations in crystal structures. The break-down of minerals and leaching of ions takes place through substitution reactions, which depend on kinetic and mechanistic fac-... 

Electrochemical window — In electrochemical experiments the range of potentials that is accessible without appreciable current flow, i.e., the potential range in which the electrode may be considered perfectly polarizable . Electrochemical windows depend on the - electrode material, the - solvent, and the - electrolyte. There is no strict definition for the current density defining the potential limits of the electrochemical window. That depends on the experiment, i.e., the signals to be measured. For highly sensitive measurements of very low current densities, the acceptable current densities at the potential limits are much smaller than in cases where high current density signals are measured. The electrochemical window also depends very much on impurities, e.g., traces of water in nonaqueous solvents, or traces of transition metal ions in aqueous electrolyte solutions. The... 

The following Table (Table 1) shows the values of the stability constants of some purines and pyrimidines with a number of transition metal ions (7, 2, 36, 69). Procedures for the synthesis of Cu, Co and Ni complexes with pyrimidines have been reported (65, 92, 93). The preparation of purine-metal compounds are described elsewhere (94, 95). There seem to be great difficulties in the quantitative study of metal complex formation, especially with the free purines, since the resulting metal chelates are almost insoluble in aqueous solution. Therefore, dioxane-water mixtures have been employed in a number of experiments. 

However, science evolves More will be seen of the quantum mechanical approach to solvation and in particular in nonaqueous solutions when there is more chance of interactions involving overlap of the orbitals of transition-metal ions and those of organic solvent molecules. Covalent bond formation enters little into the aqueous calculations because the bonding orbitals in water are taken up in the bonds to hydrogen. With organic solvents, the quantum mechanical approach to bonding may be essential. 

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