Divalent anions, alkali metals

MCAm is the ion-pair formed between the crown (C) complex containing the metal ion (Mm+) and A- is the counter ion. It needs to be noted that the degree of extraction is anion-dependent. For example, the extraction of an alkali metal into an organic phase is enhanced when the counter ion is a large anion such as picrate. Alkali metal picrates undergo extraction into benzene in the presence of 18-crown-6 in the order K+ > Rb+ > Cs+ > Na+ (Iwachido, Sadakane Toei, 1978). Divalent ions may also be extracted. For 15-crown-5 in benzene, the picrate extraction coefficients (from water) fall in the order Pb2+ > Sr2+ > Ba2+ > Ca2+... 

There are a few features relative to POMs that are necessary for obtaining the best performance. In all cases. Vanadium is present in the structure of the P/Mo Keggin anion, while the cations include different components, that is, protons, divalent transition metal ions (preferably either Fe " " or Cu " "), and alkali metal ions (preferably Cs" "). The role of Cu ions is to catalyze the reduction of molybdenum, thus increasing the activity of the catalyst it also affects the surface acidity. 

In general, precipitation at low supersaturation is performed by slow addition of mixed solutions of divalent and trivalent metal salts in the chosen ratio into a reactor containing an aqueous solution of the desired interlayer anion. A second solution of an alkali is added into the reactor simultaneously at such a rate as to maintain the pH at a selected value leading to the coprecipitation of the two metallic salts. The rate of addition can be controlled... 

The 2-(AuC1)4 and 2-(PtCl2SMe2)4 complexes (see above), show extractability properties vs. alkali metal ions, with a greater affinity for than for other alkali metal ions [48]. No structural data were available and the nature of the binding in the formation of these complexes was not investigated. Similarly, the anionic complexes [2-Cu4(/t-Cl)4(/t3-Cl)] and [2-Ag4(/t-Cl)4(/t4-Cl)] have been shown to act as host for the selective binding of alkali metal cations and divalent metal ions like or Pb. Both complexes... 

The effect of other electrolytes is also of interest. Addition of divalent cations, e.g., in the form of calcium and magnesium chloride salts, results in precipitation of the anionic vesicles, which is also observed for micelles in water. There is, however, a significant trend within the alkali metal cations, e.g., from Na+ to Rb+. The larger rubidium ion has a promotional effect on the formation of vesicles, and indeed with Cs+, vesicles are already formed on the addition of trace amounts of salt. The relevant data are shown in Fig. 19.8. 

A motif found in the majority of alkali metal stabilized carbanion crystal structures is a nearly planar four-membered ring (13) with two metal atoms (M ) and two anions (A ), i.e. dimer. This simple pattern is rarely observed unadorned as in (13), yet almost every alkali metal and alkaline earth carbanion aggregate can be built up from this basic unit The simplest possible embellishment to (13) is addition of two substituents (S) which produces a planar aggregate (14). Typically the substituents (S) in (14) are solvent molecules with heteroatoms that serve to donate a lone pair of electrons to the metal (M). Only slightly more complex than (14) is the four coordinate metal dimer (15). Often the substiments (S) in (15) are joined by a linear chain. The most common of these chains are tetramethylethylenediamine (TMEDA) or dimethoxyethane (DME) so that the spirocyclic structure (16) ensues. Alternatively the donors (S) in (16) have been observed as halide anions (X ) when the metal (M ) is a divalent cation, e.g. (17) or (18). Obviously, the chelate rings found in (16) are entropically favorable relative to monodentate donors (S) in (14), (15), (17) or (18) (Scheme 2). 

The relative charges of these inert gas core ions then determine the relative stoichiometry of the neutral ionic compounds formed, namely, AB, ABj, AB3, A2B3, etc. Some examples of such simple ionic compounds or salts are shown in Table 4.5. In these ionic salts, the inert gas cores indicate clear oxidation states that equate with the charges on the cations and anions and these must sum to zero, i.e. in AI2O3, Al "20 "3 and + 3 x 2) -H (— 2 x 3) = 0. The presence of alkali metal or alkaline earth cations in compounds such as Na2C03 or CaCOj implies that the CO3 group should be written as a discrete divalent anion, i.e. 

Other divalent cations have the tendency to form complex anions of the type [MeX3] and/or [MeX4] not only in the presence of the alkali metal halides, but also in the pure state. 

In the alkali metal pseudohalides the contribution of cationic wave functions to the valence band structure can be neglected. The optical absorption spectra can therefore be correlated to transitions involving excited states of the anions. However, one can see solid state effects like the superposition of vibronic structure on the molecular symmetry forbidden transition at 5.39 eV in the crystal spectra of the alkali metal azides (76). In the more complex heavy metal and divalent azides, a whole range of optical transitions can occur both due to crystal field effects and the enhanced contributions from cationic states to the valence band. Detailed spectral measurements on a-PbNe (80), TIN3 (57), AgNs (52), Hg(CNO)2 (72) and AgCNO (72) have been made but the level assignments can at best be described as tentative since band structure calculations on these materials are not available at present. 

Ni and CNO can exist in a metastable state in ionic lattices. Among the azides, the anion is essentially unperturbed in the alkali metal salts but in the more complex heavy metal salts increasing perturbation of the anion occurs which is reflected in the asymmetric intraionic distances of the divalent salts in particular. This may be one of the reasons why the heavy metal salts are unstable with respect to the alkali metal azides. It is therefore pertinent to note that among the divalent azides the thermal sensitivity increases with the increasing asymmetry of the azide ions which increases in the order BaNe < PbNe < CuNe (c/. Table 2). Electron microscopic observations on thallous azide crystals have shown that the cubic form of the salt is relatively stable compared with the low temperature orthorhombic form (85). This is probably associated with the existence of asymmetric azide ions in the latter poljmiorph (c/.) Section IID). 

Easy reducibility and reoxidizability by means of molecular oxygen. This allows them to be used as catalysts for liquid-phase or gas-phase multi-electron oxidations. The redox properties of these materials can be affected properly by modifying the anionic composition (for instance, by substitution of some Mo " " cations by or the cationic composition. Several different cations can be introduced in the structure, i.e. alkali and alkaline earth metals, ammonium, divalent and trivalent metals such as Cu, Co, VO. ... 

So pronounced is the chelating tendency of the diketonate anion that even alkali metal complexes may be isolated, as illustrated by RbiKCFjCOCHCOCFjljNa], in which sodium is surrounded by a trigonal prismatic array of donor oxygen atoms. For complexes derived from dibenzoylmetbane, stabilities in dioxane-water are in the order Li > Na > K > Cs. For divalent metal ions formation constants increase in the order Ba > Sr > Ca > Mg > Cd > Mn > Pb > Zn > Co, Ni, Fe > Cu, and for higher valent metal ions the first formation constants for chelates are in the order Fe + > Ga " " > Th" > In " " > Sc " " > Y " " > Sm " > Nd " > More recently, such stu-... 

Both IR and NMR data indicate that in methanol solutions solvation of both cations and anions occurs by primary solvent molecules which are strongly hydrogen-bonded to those in the bulk solvent This result has been confirmed and extended to other protic solvents through microwave investigations on alkali metal halides and salts of divalent cations in methanol, ethanol and formamide Alkali salt ion pairs in protic solvents show cation-anion distances, as found by calori-... 

As a fundamental class of compounds in the fields of synthetic solid-state (and also molecular) chemistry, cyanamides and carbodiimides have gained increasing attention within the past decade. Because of their 2-fold anionic charge, both cyanamide and carbodiimide structural units allow the realization of nitrogen-based pseudo-oxide chemistry since NCN is able to replace in a wide variety of novel materials. A number of alkali metal, alkaline-earth metal, main-group metal,divalent transition-metal, trivalent rare-earth metal,and also triva-lent transition-metal cyanamides/carbodiimides were obtained following different synthetic routes. The only carbodiimide containing divalent lanthanide ions was reported by DiSalvo et al., who found that EuNCN is isostructural to the already known a-SrNCN. ... 

Among the enniatin antibiotics, beauvericin is the one characterized in greater detail. This carrier is most interesting with respect to an anion-dependence of its transport properties Moreover, in contrast to valinomycin, it is capable of com-plexing alkaline earth as well as alkali metal ions . A study of the effects of beauvericin on the conductivity of artificial lipid membranes in the presence of both mono- and divalent cations revealed a second-order relationship between conductance and antibiotic concentration Finally, Prince, Crofts, and Steinrauf detected an apparent charge of plus one for calcium in the beauvericin-mediated transport across bacterial chromatophore membranes... 

The preferred eluents for anions are dilute carbonate-bicarbonate mixture, sodium hydroxide and, for common alkali metals and simple amines, dilute mineral acids (HCl, HNO3, BaCl2, AgN03, amino acids, alkyl and aryl sulfonic acids). The most common choice is HCl, but in the case of divalent ions, an eluent of much higher affinity for the ion-exchange resin, such as AgN03, must be used. 

Alkali metal cations and halide anions have been studied in the earlier years, but alkaline earth metal and divalent transition- and post-transition metal cations have been studied more recently. However, for divalent ions, the second ionization potential ranging from 11.02eV for Sr + through values for Ca, Mg +, Pb +, Mn +, Cr +, Zrf to 20.27 eV for Cu is larger than the ionization potential of the solvent molecules studied—water, ammonia, and methanol 12.61, 10.16, and 10.85eV respectively (except for Ca + and Sr + and water). Therefore, irreversible charge transfer M S (g) M S i(g)-i-S (g) occurs below certain minimal n values that... 

The sizes of polyatomic (nonglobular) ions in crystals are also expressed by their thermochemical radii according to Jenkins and coworkers [47], Circular reasoning may be involved in their determination, because these radii depend on calculated lattice energies of crystals that in turn depend on the interionic distances. The assigned uncertainties of these radii are 19 pm for univalent and divalent anions increasing to twice this amount for trivalent ones and they are listed in Table 2.8 too. A further problem with these values is the use of the Goldschmidt radii for the alkali metal counterions, r°, rather than the Shannon-Prewitt ones [43,44] appropriate for ions in solution. 

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