Cation/anion interactions

The interaction between a large organic anion and an organic cation may result in the formation of a relatively insoluble precipitate. 

In a study of the incompatibility of organic iodide contrast media and antihistaminic dmgs (added to reduce anaphylactic reactions) it was found that the acidity of the antihistamine solutions used caused the precipitation 

Complexes which form are not always fully active. A well-known example is the complex between neomycin sulfate and sodium lauryl sulfate that will form when Aqueous Cream BP is used as a vehicle for neomycin sulfate. Aqueous cream comprises 30% emulsifying ointment, which itself is a mixture of emulsifying wax which contains 10% of sodium lauryl sulfate or a similar anionic surfactant. 

Interactions are not always visible. The formation of visible precipitates depends to a large extent on the insolubility of the two combining species in the particular mixture and the size to which the precipitated particles grow. One might assume that in an atropine and phenobarbital mixture, a barbiturate-atropine complex may precipitate. Atropine base is soluble to the extent of 1 part in 460 parts of water. There is only 0.6 mg atropine in a 5 cm dose and it is therefore well within its solubility limit. The solubility of phenobarbital is 1 mg cm . The 15 mg of phenobarbital sodium (13.5 mg of phenobarbital) which would result if all the sodium salt were to be precipitated would be in excess of its solubility. Only 0.4 mg of phenobarbital is precipitated by 0.6 mg of atropine sulfate, however, and the phenobarbital therefore remains in solution also. There is thus no precipitation. 

Interactions between dmgs and ionic macromolecules are another potential source of problems. Heparin sodium and erythromycin lactobionate are contraindicated in admixture, as are heparin sodium and chlorpromazine 

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The structures of various salts of 8.6a have been determined by X-ray diffraction. The cation adopts a U-shaped (C2v) geometry with an bond angle of 150 1° in the absence of strong cation-anion interactions. The S-N bond lengths are ca. 1.53 A and the S-Cl distances are relatively short at 1.91-1.99 A. The structures of 8.6a, 8.7a,b and 8.8 exhibit Se-N bond lengths that are substantially shorter than the single... 

Figure 17.15 The structure of (a) the nonlinear p" cation in laAsFg and (b) the weaker cation-anion interactions along the chain (cf Fig. 17.13). For comparison, the dimensions of (c) the linear 22-electron cation L" and (d) the nonlinear 20-electron cation Te3 are given. The data for this latter species refer to the compound [K(crypt)]2Te3.en in K2Tc3 itself, where there are stronger cation-anion interactions, the dimensions are r = 280 pm and angle = 104.4°).

The analyses of the inorganic structures with the D parameter have led us to the following conclusion. The ions that have a stronger preference in their environment determine the local structure. The other ions will be arranged in close packing if the arrangement is consistent with the local structure. In other words, close packing does not determine the structure but is a consequence of the cation-anion interaction. 

The effects of substituents upon the ferroin reduction have also been recorded (Table 25) . A marked correlation between E and log A is found, indicating a single type of cation-anion interaction. 

For (Li, Cs)Cl, the internal mobilities have been calculated from Eqs. (27) and (28), and are given in Table 8. The SEVs were calculated from the same MD runs and are plotted against the calculated internal mobilities in Fig. 17 with excellent correlation between these calculated quantities. The good correlation of the SEV with the calculated and experimental internal mobilities suggests that relatively short-range cation-anion interaction plays a role in internal mobilities and the separating motion of pairs, that is dissociation, is related to the internal mobilities. In other words, the result of the SEV supports the dynamic dissociation model. 

The composition of the electrolyte is quite important in controlling the electrolytic deposition of the pertinent metal, the chemical interaction of the deposit with the electrolyte, and the electrical conductivity of the electrolyte. In the case of molten salts, the solvent cations and the solvent anions influence the electrodeposition process through the formation of complexes. The stability of these complexes determines the extent of the reversibility of the overall electroreduction process and, hence, the type of the deposit formed. By selecting a suitable mixture of solvent cations to produce a chemically stable solution with strong solute cation-anion interactions, it is possible to optimize the stability of the complexes so as to obtain the best deposition kinetics. In the case of refractory and reactive metals, the presence of a reasonably stable complex is necessary in order to yield a coherent deposition rather than a dendritic type of deposition. 

The active species of the metallocene/MAO catalyst system have now been established as being three-coordinated cationic alkyl complexes [Cp2MR] + (14-electron species). A number of cationic alkyl metallocene complexes have been synthesized with various anionic components. Some structurally characterized complexes are presented in Table 4 [75,76], These cationic Group 4 complexes are coordinatively unsaturated and often stabilized by weak interactions, such as agostic interactions, as well as by cation-anion interactions. Under polymerization conditions such weak interactions smoothly provide the metal sites for monomers. 

It appears that Te is more flexible in the possible arrangements (Figure 18) 72,73 it should be noted though that the counterions of these Te cations are more basic than the fluorinated anions used for S and Se cations. Thus, it may well be that the structural flexibility of the Te cations is a result of the increased cation-anion interactions in these salts. 

There are a number of examples known from weak-halogen bridges between T-shaped Y-Te-X complexes, like tuTe(Ph)Cl [Figure 30(a)] related weak cation-anion interaction occurs in the 2 1 complex [tu2Te(Ph)]+ Cl- [Figure 30(b)], The weak contact is generally the one trans to the Te-C bond ( trans effect of the strong Te-C bond).84,85... 

Figure 31 Cation-anion interactions in (telluroseleno)phosphonium salts...

Cation-Anion Interactions Involving Selenonium and Telluronium Salts... 

Harris has studied the complexa-tion of neomycin with pectin and demonstrated the inhibition of complex formation in the presence of an electrolyte. Potentiometric measurements indicate the mechanism of the reaction to be a cation-anion interaction. H-bonding between the hydroxy groups of pectin and sugar moieties of neomycin has been suggested and would further stabilise the compound. 

Let us first derive the regular solution model for the system AC-BC considered above. The coordination numbers for the nearest and next nearest neighbours are both assumed to be equal to z for simplicity. The number of sites in the anion and cation sub-lattice is N, and there are jzN nearest and next nearest neighbour interactions. The former are cation-anion interactions, the latter cation-cation and anion-anion interactions. A random distribution of cations and anions on each of... 

These are 13 atoms of hexagons, the central atom of which has the coordination of 12 atoms. The process of rolling flat carbon systems into NT is, apparently, determined by polarizing effects of cation-anion interactions resulting in statistic polarization of bonds in a molecule and shifting of electron density of orbitals in the direction of more electronegative atoms. 

The proportion of the /rans-O-alkylated product [101] increases in the order no ligand < 18-crown-6 < [2.2.2]-cryptand. This difference was attributed to the fact that the enolate anion in a crown-ether complex is still capable of interacting with the cation, which stabilizes conformation [96]. For the cryptate, however, cation-anion interactions are less likely and electrostatic repulsion will force the anion to adopt conformation [99], which is the same as that of the free anion in DMSO. This explanation was substantiated by the fact that the anion was found to have structure [96] in the solid state of the potassium acetoacetate complex of 18-crown-6 (Cambillau et al., 1978). Using 23Na NMR, Cornelis et al. (1978) have recently concluded that the active nucleophilic species is the ion pair formed between 18-crown-6 and sodium ethyl acetoacetate, in which Na+ is co-ordinated to both the anion and the ligand. 

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