Metal ions, solvated, displacement reactions

Solvents also play an important role in regulating the reactivity of anions. Hexamethylphosphoric triamide (HMPT) solvates alkali metal ions so strongly that the counteranions are practically free. Under such conditions the inherent bond strengths determine the ease of reaction. Since the O-C bond is stronger than the C-X bonds (X = Cl, Br, I), facile displacement of halide ions by carboxylates in HMPT is a logical consequence (35-38). 

These cations are both secondary acids, and their reactions would involve displacement, not neutralization (Chapter 8). It is customary to disregard solvation of metallic ions such as the acidic silver ion in equation 2. Yet there is no doubt that the silver ion and other acidic ions in water solution are solvated as is the proton. Apparently we may speak either of neutralization or of displacement, depending upon our point of view. This question will be discussed further in Chapter 8. 

Coordination of the anions to the cationic palladium center may strongly depend on the polarity of the reaction medium. Solvation of the ion-pair by protic solvent molecules, such as methanol, is expected to facilitate cation-anion dissociation and therefore render the metal center more electrophilic and more easily accessible for substrate molecules. In relatively apolar solvents, close-contact ion-pairs are generally expected to exist. Anion displacement by substrate molecules may then require the use of noncoordinating anions, such as certain tetraaryl borates [19], with a relatively strong affinity for interaction with the solvent molecules. This will lead to a reduced barrier for displacement of these anions by monomer molecules. 

The simulations suggest the following picture for an ion transfer reaction before the reaction the ion is located in a weak adsorption site, where it is separated from the electrode by one layer of water molecules. As the ion approaches the electrode surface it displaces water from the surface and partially looses its solvation shell this requires a substantial energy of activation. Subsequently, the ion moves down the free energy curve towards an adsorption site on the metal surface simultaneously the electronic interaction with the metal increases, electron exchange becomes adiabatic, and the adsorbed particle carries a partial charge (see next section). 

The H entry into a metal fiom an aqueous electrolyte is believed to involve the same surface-bulk transfer step as in the gas phase, but the preliminary adsorption step is a more complex process because more H sources are involved in aqueous solution, allowing more possible H surface reactions, and also because of the specificity of the electrolyte-metal interface. Whereas H adsorption in the gas phase occurs by dissociative adsorption of gaseous H2 on the free sites of a bare metallic surface, H adsorption in aqueous solution may occur either chemically by dissociation of dissolved H2 or electrochemically from solvated (hydrated) protons or water molecules it takes place on a hydrated surface and thus implies the displacement of adsorbed water molecules or specifically adsorbed ions and local reorganization of the double layer [20] competition with the adsorption of oxygen species formed from the dissociation of water may also occur [21-23], The adsorbed H layer is also in interaction with surrounding water molecules, i.e., it is hydrated [8c,24,25],... 

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