Ions, central solvate

The role of excess antimony pentafluoride can best be explained by assuming that it is capable of solvating the alkylcarbonium ion salt through interaction of the unshared electron pairs of fluorine with the vacant p-orbital of the sp -hybridized planar central carbon atom of the carbonium ion. In the system of low dielectric constant the alkylcarbonium ion is present not in the free form, but as a tightly bound ion-pair solvation affects this species rather than the free ion. 

To associate with a central ion, ligands must compete with the water molecules in (lie central ion s solvation sphere and must lose some of the water molecules in their own solvation sphere. In addition, since many ligands are the anions of weak acids, H+ competes with the central cation for the ligand. Forming a complex ion or ion pair involves competition between the cation and H+ for the ligands, and between the water, OH-, and ligands for the central cation. 

The total number of bonds with solvent and ligand molecules in the first coordination sphere is the coordination number (CN) of the ion. The first solvation number (Ns) is the number of solvent molecules in the first coordination sphere. Such a time-independent definition needs, however, a complementary time-dependent definition of the first coordination sphere. Therefore, a solvent molecule in the first coordination sphere may be defined as having a long residence time in comparison to its correlation time in subsequent coordination spheres or in the bulk. Undoubtedly, the solvation residence time varies with the lability of the metal ion. The solvation number (S) is defined as the number of solvent molecules under the influence of the electric field generated by the central ion, and following the ion s motion in the solution. 

The solute-solvent interaction in equation A2.4.19 is a measure of the solvation energy of the solute species at infinite dilution. The basic model for ionic hydration is shown in figure A2.4.3 [5] there is an iimer hydration sheath of water molecules whose orientation is essentially detemiined entirely by the field due to the central ion. The number of water molecules in this iimer sheath depends on the size and chemistry of the central ion ... 

Consider now the aquo-complexes above, and let v be the distance of the centre of mass of the water molecules constituting the iimer solvation shell from the central ion. The binding mteraction of these molecules leads to vibrations... 

In our simple model, the expression in A2.4.135 corresponds to the activation energy for a redox process in which only the interaction between the central ion and the ligands in the primary solvation shell is considered, and this only in the fonn of the totally synnnetrical vibration. In reality, the rate of the electron transfer reaction is also infiuenced by the motion of molecules in the outer solvation shell, as well as by other... 

The solvated sulfenamides [Li2( BuNSC6H4Me-4)2(THF)n] (n = 2,4) have dimeric structures with a central Li2N2 ring. The coordination mode is determined by the extent of solvation of the Li" ions monosolvation allows for rj -N,S coordination whereas disolvation restricts the coordination mode to // -M Variable temperature NMR studies indicated that a dynamic exchange between these two structural types occurs in THF solution (Scheme 10.10). The dihapto coordination mode is observed exclusively in transition-metal complexes and the... 

In practical applications of this equation, one must pick values for constant a. To a first approximation it can be regarded as equal to the sum of the radii of two solvated ions. It is not clear, however, whether the solvation sheaths of approaching ions would not be deformed. Moreover, in deriving Eq. (7.43) it was assnmed without sufficient reasoning that the constant a for a given central ion will be the same for different ions present in the ionic atmosphere. 

The structure of the ions, where the bulky phenyl groups surround the central ion in a tetrahedron, lends validity to the assumption that the interaction of the shell of the ions with the environment is van der Waals in nature and identical for both ions, while the interaction of the ionic charge with the environment can be described by the Born approximation (see Section 1.2), leading to identical solvation energies for the anion and cation. 

Let us denote by R the coordinates of all the nuclei involved, those of the central ion, its ligands, and the surrounding solvation sphere, and by r the coordinates of all electrons. The Hamiltonian for the... 

It should be noted that the local composition model is not consistent with the commonly accepted solvation theory. According to the solvation theory, ionic species are completely solvated by solvent molecules. In other words, the local mole fraction of solvent molecules around a central ion is unity. This becomes unrealistic when applied to high concentration electrolyte systems since the number of solvent molecules will be insufficient to completely solvate ions. With the local composition model, all ions are, effectively, completely surrounded by solvent molecules in dilute electrolyte systems and only partially surrounded by solvent molecules in high concentration electrolyte systems. Therefore, the local composition model is believed to be closer to the physical reality than the solvation theory. 

Many attempts have been made to calculate important macroscopic properties of ionic solutions without a detailed knowledge of their molecular structure. All models developed for this purpose can be classified into three categories according to the way the surrounding of the central ion is accounted for. Usually, continuous and discontinuous theories of ionic solvation are distinguished. Furthermore, it seems appropriate also to mention various attempts to apply statistical mechanics directly to ionic solutions. 

Electron transfer reactions, treated by continuum theory, suggested that the Franck-Condon barrier (the barrier for the vertical transition of electrons), which is about four times the activation barrier for the isotopic electron transfer in solution, is due to Bom continuum solvation processes. Specific contributions for the activation of ions come from the solvent continuum far from the ion the important contribution from the solvent molecules oriented toward the central ion in the first and second solvation shells is neglected. ... 

An extensive review appeared on the configurational stability of enantiomeric organolithium reagents and the transfer of the steric information in their reactions. From the point of view of the present chapter an important factor that can be evaluated is the ease by which an inversion of configuration takes place at the metallation site. It happens that H, Li, C and P NMR spectra of diastereotopic species have been central to our understanding of the epimerization mechanism depicted in equation 26, where C and epi-C represent the solvated complex of one chiral species and its epimer, respectively. It has been postulated that inversion of configuration at the Li attachment site takes place when a solvent-separated ion pair is formed. This leads to planarization of the carbanion, its rotation and recombination to form the C—Li bond, as shown in equation 27, where Li+-L is the solvated lithium cation. An alternative route for epimerization is a series of... 

A specific synthesis of the 4,5-diphenyl 1,2,3-triphospholide ion (206) starts from the tetraphenyl dihydro-1,2-diphosphete (205) and involves several steps (Scheme 60). In the last step probably at first the central P—P bond of the bis(dihydrotriphospholyl) is cleaved and subsequently the P—Ph bonds. The potassium salt is solvated by DME and isolated as a white powder <93PS(77)254, 94BSF397). 

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