Solvation of Alkali-metal Cations

The majority of data published on the solvation (both aqueous and non-aqueous) of alkali-(and alkaline-earth-)metal cations is of but peripheral interest to the inorganic chemist. Consequently, the papers abstracted for this section of the Report are quite selective, dealing principally with the structural and spectroscopic properties of these solutions. 

As a starting point in a theoretical study of ionic solutions, the complex H2O-Li -F has been considered. Analysis of the stabilization energies of some 250 geometrical configurations reveals the existence of at least three possible structures (i) the Li-F-H20 structure that has C2 symmetry (ii) a second Li-F-H20 structure with the F forming a hydrogen bond (with a hydroxy-group) and (iii) the F-Li-H20 structure that has C2V symmetry.  

The structures of these ionic solutions have been studied, using X-ray diffraction, n.m.r., and ultrasonic techniques. X-Ray diffraction measurements of aqueous Nal solutions showed that the Na ion is bonded to ca. four water molecules at a Na - O distance of ca. 0.24 nm. Similar experimental data for aqueous CaBra solutions can be rationalized with both six- and eight-fold co-ordinate Ca ions. In both solutions, the halide ion is approximately octahedr-ally co-ordinated.  

Studies of Li n.m.r. relaxation times of aqueous LilOa solutions containing added iodic acid or iodates have established that, up to concentrations of LilOa of 3 mol r , the IO3 ion does not substitute in the first hydration shell of the Li ion. 

deWitte, R. C. Schoening, and A. I. Popov, Inorg. Nuclear Chem. Letters, 1976, 12, 251. 

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In an extensive series of papers Popov and his collaborators have studied the solvation of alkali metal cations in various nonaqueous media, using Na magnetic resonance shifts (66), and far-infrared spectroscopy. The ion-solvent vibrational bands... 

A review of nuclear magnetic properties of alkali metal nuclei is followed by a brief survey of solvation of alkali metal cations, their hydration especially. The chemistry of alkali metal anions is then evoked. Chemical shifts and relaxation rates will be described with emphasis on the predominant factors contributing to these observables. A brief history of alkali metal NMR will be followed by selected applications to ion pairing phenomena, inclusion complexes, preferential solvation, the chelate effect, and polyelectrolytes. 

SOLVATION OF ALKALI METAL CATIONS Geometric Considerations... 

Furthermore, the equivalent conductivity is known to decrease with concentration as c1/2 for dilute solutions (Kohlrausch law). At higher concentrations the conductivity usually increases above the Kohlrausch law value [107]. Furthermore, in weakly polar solvents, there is extensive evidence that strong electrolytes do not dissociate completely, but neutral ion pairs remain in solution [107]. Indeed, solutions of alkali metals in ethers have received considerable attention and two forms of alkali-metal-cation—solvated electron ion pair have been characterised by Seddon et al. [108]. Reactions of an ion as an ion or when ion-paired should be considered as two totally different processes. 

Dissolution of alkali metal cations such as Cs+ results in short-range liquid order in water as a primary solvation shell of about eight water molecules is established about the metal cation. Lithium, however, exerts a much greater polarising power and is capable of organising a first- and second-coordination sphere of about 12 water molecules about itself, resulting in a much larger hydrated radius for the ion and hence decreased ionic mobility. 

There is also a large group of ion-conducting organic polymer electrolytes (typically containing solvent or low-molecular-weight plasticizer) that are capable of solvating dissolved alkali metal cations (e.g., Li+). 

Properties of the complexes of alkali metal cations with various bases are important in understanding ion-molecule interactions, solvation effects, biomedical and physiological phenomena related to ion channels and relevant in medical treatments. Reliable experimental bond dissociation enthalpies, and thereby gas-phase alkali ion affinities, could now be obtained using various mass spectrometry techniques such as the Fourier-transform ion cyclotron resonance (FT-ICR), collision-induced dissociation and photodissociation methods. However, these methods do not provide direct information on the adduct structures. 

Table 20 Solvation Factors Promoting Selectivity for Cs Ion in the Cation-Exchange Extraction of Alkali Metal Cations from Water to Organic Solvents...

Several organic solvents, notably aliphatic amines and ethers are stable over prolonged periods towards solvated electrons, which can be generated cathodically by the discharge of alkali metal cations. The characteristic blue colour of the solvated electron is readily formed near cathodes... 

Many of the early studies focused on the thermochemistry of hydration and ammoniation of alkali metal cations, Ag+, HjO" ", and NH. These studies focused initially on the value of such data in understanding nucleation and gas-phase ion solvation phenomena but were quickly appreciated as probes of solvation energies in the bulk solvent. 

A study of i.r. vibrations of alkali-metal cations encased in dibenzo-18-crown-6 reveals that the ions Na+ and K+ have about equal complexation forces, and cation selectivity therefore stems entirely from the difference in stability of the solvated cations. I.r. methods have also been used in characterizing stable H30+-polyether complexes formed in aqueous perchloric acid, and X-ray diffraction studies of barium thiocyanate complex with isomer A of dicyclohexyl-18-crown-6 (obtained with isomer B on hydrogenation of dibenzo-18-crown-6) show that this isomer has the cis-syn-cis configuration. ... 

Crown ethers have low soluhiUty in water hut have considerable solubility in organic solvents. Crown ether complexes of alkali metal cations are therefore quite stable in organic phases. The anions of the alkali metal cations present in the aqueous phase, Y, are simultaneously extracted into the organic phase the complex in the organic phase may be represented as (crown-M)+ y. Due to the likely absence of solvation of such anions in the organic phase, they are likely to be highly reactive. Nevertheless, crown ethers and similar host compounds have been successfully incorporated into organic solvents to extract aUcali metal ions/salts from aqueous solutions. 

The importance of alkali metal binding with available 7r-electron density in the formation of CIPs was also demonstrated by Niemeyer in the structural elucidation of the first monomeric non-solvated lithium cuprate, [(2,6-Mcs2(LI L)2CuLi] 450, formed from the reaction of 2 equiv. of (2,6-Mcs2Gf,I L)Li with /-BuOCu in pentane.447 The complex crystallizes as two different independent molecules in which the C-Cu-C angles differ (171.1° and 173.8°) as does the mode of coordination to the Li cations C pso and rf to one pendant Ph in molecule 1, with an additional rf interaction to a second Ph group in molecule 2. In the second molecule, the Li site is 10% occupied by a Cu ion. 

Sulfur diimides react quantitatively with organolithium reagents at the sulfur centre to produce lithium sulfinimidinates of the type Li[RS(NR )2] A. The lithium derivatives may be hydrolysed by water to R NS(R)NHR which, upon treatment with MH (M=Na, K) or the metal (M=Rb, Cs) in THF, produces the heavier alkali-metal derivatives.132 The structures of these complexes are influenced by (a) the size and electronic properties of the R group, (b) the size of the alkali metal cation, and (c) solvation of the alkali-metal cation. 

Lariat ethers of structure 8 were found to be selective toward Li ion and the lariat crown ether-Li+ complexes are more stable than the corresponding complexes with Na or K+, in methanol. Nevertheless, experiments conducted in aqueous solution showed that Na+ had a better complexation ability than the other two alkali metal cations. Hence, selective complexation of lariat crown ethers with cations changes with the solvent system this may be due in part to the difference in solvation between solvent and cation (Figure 9 f. ... 

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