Equilibrium with metal ions, solvated electrons

Solvated electrons in ammonia are formed in equilibrium with metal ions dissolved in this medium (76). Analogous behavior was reported for ethylenediamine (42). On mixing ethylenediamine solutions of alkali metals with water, hydrated electrons were claimed to he formed as transients (43). 

A similar situation may be obtained when alkali metals are immersed in ultrapure ethers containing benzophenone [53], The metal thus dissolves via formation of stable ketal radical anions in solution (and metal ions as well). It should be emphasized that the above processes occur even when the active metal is initially introduced into the solution covered by surface films (due to reactions with atmospheric contaminants). We assume that electron tunneling through the films enables the initiation of the dissolution process. This process breaks the film on the metal (as metal is depleted beneath the rigid surface film), thus enabling solvent molecules to reach the active surface and solvate more electrons. This increases the metal solubilization and the further breakdown of the initial surface films. Hence, an equilibrium between a bare metal and the blue solution can finally be reached, as explained above (Eq. 13). 

The following observation emphasizes the influence of the temperature on ion-solvation equilibrium. The reduction product of 1 with lithium metal in methyltetrahydrofuran is temperature-dependent11. At —120 °C only the radical anion (1 ) could be observed by ESR, while at higher temperatures the paramagnetism disappears and the dianion (12 ) is detected. This reaction must be endothermic it therefore seems that disproportionation is driven by entropy and not by energy, due to ion-pair-solvation equilibrium. It is noteworthy that 12 cannot be observed by NMR spectroscopy due to its special electronic structure12. 

The second equilibrium involves hydrated 1 ions in equilibrium with E" complexes. In the forward step, an iodide ion donates an electron to the working electrode and is hence oxidized to a I atom. We may speculate that the 1 ion remains outside the double layer and that an electron tunnels through the double layer. However, from many experimental results and molecular dynamic simulations (see Section 4.7.2), it became clear that this is not the case. Instead, a solvated ion penetrates the double layer and becomes chemisorbed as a 1" ion (<5 < 1) on the metal surface, losing about half of its hydration shell [18, 19]. Moreover, there is a local restructuring of the double layer. Here also, the electrochemical reaction does not involve tunneling of an electron through the double layer. 

The solvated electron in liquid ammonia was discovered in 1864 by Weyl, and was identified in 1908 by Kraus as an electron, e am, in a cavity surrounded by ammonia molecules. It is prepared when an alkali metal, for example sodium, is dissolved in ammonia, to form a stable blue color under these conditions the electron is present in equilibrium with the metal atom and cation [34]. By contrast, e am produced by pulse radiolysis of liquid ammonia is unstable due to its reactions with other radiolysis products for example, in pure ammonia at —45 °C, its lifetime is ca. 7 ps [35], The major decay reactions are thought to involve the oxidizing radicals NH2 and NH [36], because addition of potassium ethoxide stabilizes e am this is explicable, since ethoxide ion is expected to scavenge these radicals [36a]. 

When a metal electrode is placed in an electrolyte solution, an equilibrium difference usually becomes established between the metal and solution. Equilibrium is reached when the electrons left in the metal contribute to the formation of a layer of ions whose charge is equal and opposite to that of the cations in solution at the interface. The positive charges of cations in the solution and the negative charges of electrons in the metal electrode form the electrical double layer [4]. The solution side of the double layer is made up of several layers as shown in Fig. 2.7. The inner layer, which is closest to the electrode, consists of solvent and other ions, which are called specifically adsorbed ions. This inner layer is called the compact Helmholtz layer, and the locus of the electrical centers of this inner layer is called the inner Helmholtz plane, which is at a distance of di from the metal electrode surface. The solvated ion can approach the electrode only to a distance d2. The locus of the centers of the nearest solvated ion is called the outer Helmholtz plane. The interaction of the solvated ion with metal electrode only involves electrostatic force and is independent of the chemical properties of the ions. These ions are called non-specifically adsorbed ions. These ions are distributed in the 3D region called diffusion layer whose thickness depends on the ionic concentration in the electrolyte. The structure of the double layer affects the rate of electrode reactions. 

At the beginning of the nineteenth century, the British chemist Davy first prepared alkali metal-ammonia solutions. Davy was also the first person to make alkali metals using electrochemistry. Ammonia condenses at -33.35°C and becomes solid at -77.7°C. Alkali and some other metals dissolve readily in anhydrous ammonia solutions. Already in 1908, on the basis of conductivity measurements, Kraus proposed that alkali ions A+ exist in ammonia together with cavities containing a single electron (solvated electrons), in equilibrium with dissolved alkali metal atoms. At not too high concentration, alkali solutions are all deep blue, suggesting that the color arises from the electron cavities rather than directly from the metal ion. 

These centres are formed by the addition of monomer to a suitable anion. They are almost always simpler than their cationic reverse part. The counter ion is usually a metal cation able to interact with the electrons of the growing end of the macromolecule, and to bind in its ligand sphere monomer or solvent molecules or parts of the polymer chain. This changes the properties of the whole centre. Therefore, by selection of the metal, the stability of the centre, the tendency of the centres to aggregation, the position of the equilibrium between the contact and solvent-separated ion pairs and free ions, and the stereoselectivity of the centre [the ability to produce polymers with an ordered structure (tacticity, see Chap. 5, Sect. 4.1)] are predetermined. The chemical reactions of the metal cations are, however, very limited. Most solvents and potential impurities are of nucleophilic character. They readily solvate the cation, leaving the anion relatively free. The determination... 

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