Cations solvated

The layer of solvent molecules not directly adjacent to the metal is the closest distance of approach of solvated cations. Since the enthalpy of solvation of cations is nomially substantially larger than that of anions, it is nomially expected that tiiere will be insufBcient energy to strip the cations of their iimer solvation sheaths, and a second imaginary plane can be drawn tlirough the centres of the solvated cations. This second plane is temied the outer Helmholtz plane (OHP). 

Fig. 1. The structure of the electrical double layer where Q represents the solvent CD, specifically adsorbed anions 0, anions and (D, cations. The inner Helmholtz plane (IHP) is the center of specifically adsorbed ions. The outer Helmholtz plane (OHP) is the closest point of approach for solvated cations or molecules. O, the corresponding electric potential across the double layer, is also shown.

Particularly striking examples of the effect of specific solvation can be cited from among the crown ethers. These are maciocyclic polyethers that have the property of specifically solvating cations such as Na+ and K" ". 

Polar aprotic solvent (Section 11.3) A polar solvent that can t function as a hydrogen ion donor. Polar aprotic solvents such as dimethyl sulfoxide (DMSO) and dimethyl-formamide (DMF) are particularly useful in Sn2 reactions because of their ability to solvate cations. 

Polar aprotic solvents dissolve ionic compounds, and they solvate cations very well. 

Polar protic solvents solvate cations and anions effecttively. 

The lack of reactivity, coupled with the ability of ethers to solvate cations makes... 

Sodium hydroxide in solution dissociates to yield solvated cations and anions, Na+ and the hydroxide ion OH- respectively ... 

With physically meaningful ionic radii at hand, it is a rather simple task to evaluate the electrostatic properties of solvated cations in the frame of Bom formalism. For instance, we may redefine the net charge of the ion given in Eq (2) as follows ... 

The paper contains several innovations in the interpretation of polarographic reduction potentials, Eyi. One is the recognition that the Eyi obtained in the presence of base-electrolytes is not that of an isolated, solvent-solvated cation, but of one which is part of an ion-pair or of a higher aggregate. A practically useful innovation is to use the Em of the triphenylmethylium ion as the zero of the potential scale in all solvents. By means of this device one can compare a wide range of Ey2 differences in different solvents, and it is especially useful because that ion is stable in strongly acidic media in which the commonly used marker ferrocene decomposes. 

The monomer-solvated cation Pn+M. Although this species first appeared in 1948, its importance was not realised until comparatively recently. Provided that its... 

The second argument advanced by Penczek and Matyjaszewski was that in PhN02 solvent there is an equilibrium between the normally solvated cation PB+ and the cation covalently bound to the solvent, P +-ON(0)Ph,... 

A key feature of the mechanism of Wilkinson s catalyst is that catalysis begins with reaction of the solvated catalyst, RhCl(PPh3)2S (S=solvent), and H2 to form a solvated dihydride Rh(H)2Cl(PPh3)2S [1], In a subsequent step the alkene binds to the catalyst and then is transformed into product via migratory insertion and reductive elimination steps. Schrock and Osborn investigated solvated cationic complexes [M(PR3)2S2]+ (M=Rh, Ir and S= solvent) that are closely related to Wilkinson s catalyst. Similarly to Wilkinson s catalyst, the mechanistic sequence proposed by Schrock and Osborn features initial reaction of the catalyst with H2 followed by reaction of the dihydride with alkene for the case of monophosphine-ligated rhodium and iridium catalysts [12-17]. Such mechanisms commonly are characterized... 

Rates are found to be a factor of ten slower than diffusion control suggesting a requirement for solvent reorganization around the strongly solvated cations. 

The rate at which solvent molecules are exchanged between the primary solvation shell of a cation and the bulk solvent is of primary importance in the kinetics of complex formation from aquocations. In both water exchange and complex formation, a solvent molecule in the solvated cation is replaced with a new molecule (another water molecule or a ligand). Therefore, strong correlations exist between the kinetics and mechanisms of the two types of reactions. 

A common technique for measuring the values has been to employ species that produce anions with useful ultraviolet (UV) or visible (vis) absorbances and then determine the concentrations of these species spectropho tome trie ally. Alternatively, NMR measurements could be employed, but generally they require higher concentrations than the spectrophotometric methods. A hidden assumption in Eq. 5 is that the carbanion is fully dissociated in solution to give a free anion. Of course, most simple salts do fully dissociate in aqueous solution, but this is not necessarily true in the less polar solvents that are typical employed with carbanion salts. For example, dissociation is commonly observed for potassium salts of carbanions in DMSO because the solvent has an exceptionally large dielectric constant (s = 46.7) and solvates cations very well, whereas dissociation occurs to a small extent in common solvents such as DME and THE (dielectric constants of 7.2 and 7.6, respectively). In these situations, the counterion, M+, plays a role in the measurements because it is the relative stability of the ion pairs that determines the position of the equilibrium constant (Eq. 6). 

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