Ion triplet

Cryoscopic investigations suggest that in sulpholan nitronium tetrafluoroborate exists predominantly as ion pairs. - The specific conductivity of these solutions increases linearly with the concentration of the salt (up to 0-4 mol 1 ), and is attributed to the existence of ion-triplets rather than free ions. ... 

It is proposed that one pre-equilibrium gives the ion-pair [KFe(CN)5] which may form an ion-triplet in a second fast pre-equilibrium, viz. 

The ion-triplet oxidises I in the slow step to yield -Iz . The non-participation of any ligand substitution step is confirmed by the absence of any incorporation of activity from added C-labelled free cyanide ion into the product ferrocyanide ... 

Tomlinson, E. Davis, S. S., Interactions between large organic ions of opposite and unequal charge. II. Ion pair and ion triplet formation, J. Colloid Interface Sci. 74,349-357 (1980). 

The species appearing as strong electrolytes in aqueous solutions lose this property in low-permittivity solvents. The ion-pair formation converts them to a sort of weak electrolyte. In solvents of very low-permittivity (dioxan, benzene) even ion triplets and quadruplets are formed. 

HOO" and RO" are not necessarily adjacent to each other in the intermediate ion triplet. Thus, the solvation shell of 1-octanol could help transfer the proton. 

Consequently, all the data in Figure 2 can be explained in terms of the different reactivities of the various aggregates. The insensitivity of the rate of thiocyanate entry to the thiocyanate concentration simply shows that, over the whole concentration range studied, the substrate was always in the form of the ion triplet. The rate dependence on the chloride or bromide concentration represents the change in the distribution of the substrate between the 2 1 and the 3 1 aggregate as the anion concentration is increased. 

The kinetic data reveal a complex dependence on the anion concentration and the hydrogen-ion concentration and have been interpreted on the basis of ion-pair and ion-triplet formation. The uncatalyzed path ( 0)has been shown to involve (NH3)5Co(OH)Co(NH3)55+ (= M5+)and the ion pair M x Y4 +, and it was proposed that the ion pair M x Y4+ scavenges Y- from solution and not from the second coordination sphere (357). It was shown that the reactive intermediates are quite selective for anions (as well as being selective for the N terminus of NCS-, the ratio for N-bound S-bound being approximately 4), and this has been interpreted as arising from a genuine, coordinately unsaturated intermediate. The acid-catalyzed path has been interpreted in terms of the formation of protonated unaggregated reactant, MH6+, and small concentrations of the protonated ion pairs and ion triplets MH x Y5+ and MH x Y24+ (355, 356). 

The second equilibrium ion-pah acidity constant (pKi) of 9,10-dihydrodibenz[a,/i]-anthracene (DBDHA) (2) in THF at 298 K has been found to be considerably lower than for other 9,10-dihydroanthracene (DHA) derivatives and the dependence on the counter ions sodium (pAA 28.5) and potassium (pAA 30.4) is less marked as a consequence of extended p. re-conjugation of the dianion (3) with the outer benzene rings.5 The disodium, dipotassium, and dirubidium salts exist as contact ion triplets and the dilithium salt as a solvent-separated triplet. 

The tube-like receptor 39 was shown to bind two imidazolidin-2-one molecules inside the calixarene subunits or, more interestingly, ion triplets comprising an anion, which binds to the central urea moieties, and two ammonium ions, which are incorporated into the calixarene rings [86]. Particularly stable complexes are formed with ammonium sulfate salts even in relative polar media (CD3OD/ CDCI3 3 1), but chloride ions can also induce the formation of such complexes... 

Very surprisingly, anhydrous copper(II) nitrate, and to a lesser extent eopper(II) perchlorate, are volatile. They may be sublimed in vacuo at about 200° C and probably exist as ion triplets of the type... 

Table XV.5 shows the rather dramatic change in Xeq for the dissociation of tetrisoamyl ammonium nitrate, (i-Am4N)+N03 ", with dielectric constant in mixtures of H2O and dioxane. Although it is possible to get much better agreement with the conductance data by using slightly different values of the case shown is used to emphasize the essential correctness of the method. Note also that no account has been taken of the preferential solvation of ions by one of the two solvents. The Fuoss and Kraus treatment also gives a simple model for the calculation of ion triplet and quadruplet concentrations.

The range of accessible dielectric constant is somewhat limited by the fact that in low-dielectric-constant media, ion pairing begins to become important, and the contribution of ion triplets and quadruplets may also become important. In addition, selective solvation of the ions by the higher-dielectric-constant solvent will here assume its most important aspects. Little information is available on this point, but the kinetic data do show significant deviations from straight lines in the In kn vs. 1/D plots at low D. 

Fig. 4.106. The V2(N02) stretching region of the Raman spectra of sodium nitrite solutions at about 1.0 Min liquid ammonia at 293 K. The second spectmm isoneofasolutbn r which [NaCI04]/[NaN02]=4. Band A, either ion triplet [Na NOj-Na ] or contact ion pair containing bidentate NOj bands B and C, contact ion pairs [Na N03°and [Na ONO ]° band E, tree solvated anion [N02(NHg)J" (P. Gans and J. B. Gill, J. Chem. Soc. Faraday Discuss. 64 A 50, 1978).

When the dielectric constant of the nonaqueous solvent goes below about 15, ions can associate not only in ion pairs but also in ion triplets. This comes about by one of the ions (e.g., M )ofan ion pairM -A Coulombically attracting a free ion A strongly enough to overcome the thermal forces of dissociation... 

Nevertheless, minima in plots of A vs. Ic do not unambiguously prove the existence of triple ions. Grigo showed that it is also possible to reproduce the A data for sodium iodide in butanol in terms of dielectric constant variation without the assumption of ion triplets. In summary, doubts still exist in relation to the formation of triple ions. Such doubts are most likely to be relieved by information that may become available through Raman and other kinds of spectroscopy. 

The term AmG (y,I) is the Increase in excess free energy due to the mixing of the two solutions, and represents the changes in molecular configuration, such as the formation of ion-pairs, ion triplets, etc. 

The interpretation of the branch, where the WP increases, more sophisticated. A minimum of WP implies a redissociation of the pairs and/or the formation of charged ion clusters. Fuoss and Kraus [41] assumed the formation of charged ion triplets. Recent theory attributes the increase of the conductivity to redissociation, resulting from the interactions of the free ions with the ion pairs and the increase of the dielectric permittivity due to the formation of ion pairs that causes a decrease of the association constant [38],... 

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