Existence of Complex Ions

3 Experiments on Complex Reactions E9.1 Existence of Complex Ions 

Problem Students know the ionic symbols like Na+(aq) and Cl (aq) with the (aq)-symbol one will show that about four to six water molecules are surrounding one ion attached by electrostatic forces. With a simple experiment about the dissolving process of three different copper salts, the student will conclude that the Cu2 + (aq) is responsible for the same blue color of all three solutions. Adding ammonia solution to all three liquids, an identical deep-violet solution results. What particles are now responsible for this new color The teacher has to introduce copper complexes, symbolized by special symbols [Cu(H20)6]2+ and [Cu(NH3)4(H20)2]2+. These six ligands in both complexes combine with the central ion by a special structure which can be shown by octahedron models (see Figs. 9.2 and 9.9). 

Material Test tubes white copper sulfate, green copper chloride, black copper bromide, water, concentrated ammonia solution. 

Procedure To each of the three copper salts, add a little amount of water shake the test tubes. In a second step, add a few mis of ammonia solution to all three solutions. 

Observation Although the salts initially have different colors, all the three solutions are bright blue. They turn into the same deep violet color after adding ammonia solution. 

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ICotschubei6 has determined the extent of the hydration of the cobalt ion in cobalt chloride solution by the electrolytic method briefly indicated in the first volume of this Series,6 and finds that the hydration increases with the dilution. He concludes from his own and other workers experiments that the hydration diminishes with rise of temperature also that the hydration of the cobalt chloride molecule varies in the same manner as that of the cobalt ion. He doubts the existence of complex ions in solutions of cobalt chloride, and considers that in the blue solutions formed by the addition of hydrochloric acid, calcium chloride, etc., the evidence for the existence of complex ions is inconclusive. On the other hand, he admits the presence of complex... 

Until the 1980s, the major methods of investigation in ionics were nonspec-troscopic. For example, conductance results were used to infer the existence of complex ions. Alternatively and typically, the change of the dielectric constant of a solution as a function of the concentration of ions (measurements at various frequencies) was interpreted in terms of structural hypotheses about ion-solvent interactions. 

For the ionic liquids, however, without a separating solvent, the situation is different for the ions are always in contact. This absence of solvent causes conceptual problems regarding the existence of complex ions in ionic liquids. Consider a particular ion associated with another to form a vibrating complex. The ion is also in contact with, and continually jostled by, neighboring ions that are exactly like its partner in the complex (Fig. 5.4). Which is the partner and which the neighbor Which is the vibration and which the collision A distinction between these two types of contacts constitutes one of the problems in this field. 

Such problems can be tackled by spectroscopic means, as shown later. Raman spectra, in particular, would indicate new lines having characteristic frequencies when Br is added to CdlNOjjj in KNOj-LiNOj, and in the preceding section it has been shown that an analysis of the variations of the electrode potential of Cd(N03)2in KNOj-LiNOjWith Cr addition has given reason to believe in complex ions in the cases quoted. However, there is a nifty electrochemical method that allows one to also obtain the lifetime of the individual ions and hence remove doubt as to the real existence of complex ions in molten salts. 

The most direct proof of the existence of complex ions was obtained using spectroscopic measurements. The presence of [CoCU], [NiCl4] , [CuCl4] , [PbCl3] , and... 

Ostwald (2) combined the Arrhenius theory of electrolytic dissociation with the law of mass action and calculated the dissociation constants of various weak acids from the results of conductivity measurements. The existence of complex ions could be deduced from distribution experiments (3) and solubility behavior (4) as well as from rate studies, and several equilibrium constants were determined. 

The formation of acyl halide-Lewis acid complexes have been observed by several methods. For example, both 1 1 and 1 2 complexes of acetyl chloride, with AICI3 can be observed by NMR spectroscopy. The existence of acylium ions has been demonstrated by X-ray diffraction studies on crystalline salts. For example, crystal structure determinations have been reported for /i-methylphenylacylium and acetylium ions as SbFg salts. There is also a good deal of evidence from NMR measurements which demonstrates that acylium ions can exist in nonnucleophilic solvents. " The positive charge on acylium ions is delocalized onto the oxygen atom. This delocalization is demonstrated in particular by the short O—C bond lengths in acylium ions, which imply a major contribution from the structure having a triple bond ... 

Several groups confirmed spectroscopically the existence of an ion-pair complex Fe -HOJ, which may decompose unimolecularly at high [H2O2]/ [Fe(III)] but become transformed to FeO " " at lower [H202]/[Fe(III)]. FeO " is attacked by a second molecule of H2O2 to provide an alternative route for decomposition, viz. 

Measurements of the electrical conductivity and solubility of difficultly soluble salts in sodium tetrametaphosphate solutions 62, 151, 196, 197) and potentiometric titrations lead to the assumption of the existence of complexes of the type of Na+(P40i2)3 Ba"1-1", Sr, Ca++, Mg++, Mn++, Ni++, Cu++(PiO]2) - La(P40i2) Cu(P40i2)26- and Ni(P40i2)26. No comparable dissociation constants for these have so far been given, though in any case they will be smaller than for the corresponding ion pairs of the trimetaphosphate anion 145). 

The existence of isolated ions of high charge such as Ti 4 in chemical systems b energetically unfavorable. Nevertheless, complexes exist with these elements in high formal oxidation states. 

The Ni3S2 fragments in the two complexes (263) and (265) have a trigonal bipyramidal geometry with the two S atoms in the apical positions and the three nickel atoms in the equatorial plane, as found in other organometaUic compounds which contain the same Ni unit.1939 The formation of the enneanuclear complex (264), on the contrary, is exceptional and no other complex of this stoichiometry was isolated with analogous tertiary phosphines, with selenium, or with metals other than nickel. It is not possible to assign any definite oxidation number to nickel in complex (264). The lack of two electrons with respect to the situation of nine nickel(II) ions was reported to be essential for the existence of complex (264) because the oxidation is spontaneous and its reduction invariably leads to the decomposition of the cluster compound.19 ... 

From studies of the reaction between silver and thiosulfate ions, a large number of complex ions have been suggested to exist in solution. The available thermodynamic data for these species are collected in Table 33.239,240... 

Sterten [33] used activity data and calculated the concentration of complex ions in cryolitic melts saturated with alumina and the distribution of anions as a function of the molar cryolite ratio (NaF/AlF3), as shown in Figure 5. Julsrud [34] and Kvande [35] suggested the existence of some ions of Al2OF84 for electrolytes with cryolitic ratio = 3, while the A1202F42 ions were in majority in... 

Note that milk contains considerably more cations than anions Jenness and Patton (1959) have suggested that this can be explained by assuming the formation of complex ions of calcium and magnesium with the weak acids. In the case of citrate (symbol ) the following equilibria exist ... 

Third, there exists another problem, that of complex ions. In aqueous and nonaqueous solutions, it is possible to regard the ionic atmosphere as a type of incipient complex in which the mean distance between oppositely charged ions becomes smaller with increasing electrolyte concentration. Eventually the ions come sufficiently close so that the thermal forces that tend to separate them are overcome on an increasingly frequent basis by the Coulombic attraction forces so that cation and anion pairs arise, some of which remain stuck together (see Section 3.8). 

Consider complex ion formation in the CdClj-KCl system, and let it be assumed for the moment that a CdCl complex ion is formed. If such complex ions were formed in an aqueous solution of CdClj and KCl, they would exist as little islands separated from other ions by large expanses of water. In fused salts, there are no oceans of solvent separating the ions. Thus, a Cd " ion would constantly be coming into contact on all sides with chloride ions, and yet one singles out three of these CP ions and says that they are part of (or belong to) a CdCIJ complex ion (Fig. 5.54). It appears that in the absence of the separateness possible in aqueous solutions, the concept of complex ions in molten salts is suspect As will be argued later, however, what is dubious turns out to be not the concept but the comparison of complex formation in fused salts with complex formation in aqueous solutions. 

From the standpoint of this comparison (Fig. 5.54), it is seen that the concept of a complex ion in a molten salt is at least as tenable as that of an ion with a primary solvation sheath (Section 2.4) in aqueous solutions. Whatexperimental evidence exists for complex ions in fused salt mixtures To anwer this question, one must discuss some results of investigating the structure of mixtures of simple ionic liquids. 

Ideas on complex ions in molten salts tend to vary with the time at which they were published. In the first half of the century, there seemed no doubt that complex ions in molten salts were distinct entities and, it was implied, they were permanent. Later, there was doubt as to our ability to identify complex ions in molten salts. Thus, it was argued, there is no difficulty in accepting the existence of discrete ions in aqueous solutions because each ion is a separate entity, and there are many solvent... 

Analytical Difficulties Although we can make many chemical equilibrium models that predict the existence of complexes in natural waters, analytically one encounters difficulties in identifying unequivocally the various solute species and in distinguishing between dissolved and particulate concentrations. The analytical task is rendered veiy difficult because the individual chemical species are often present at nano- and picomolar concentrations. The ion-selective electrode (ISE), if it were sufficiently sensitive, would permit the measurement of free metal-ion activity. 

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