Thallium redox reactions

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

An attempt to oxidise vanillin (3-methoxy-4-hydroxybenzaldehyde) was made using thallium trinitrate and when formic acid at SO C as a solvent was present. The extremely violent redox reaction that followed was put down to the effect of salt on acid. 

The participation of cations in redox reactions of metal hexacyanoferrates provides a unique opportunity for the development of chemical sensors for non-electroactive ions. The development of sensors for thallium (Tl+) [15], cesium (Cs+) [34], and potassium (K+) [35, 36] pioneered analytical applications of metal hexacyanoferrates (Table 13.1). Later, a number of cationic analytes were enlarged, including ammonium (NH4+) [37], rubidium (Rb+) [38], and even other mono- and divalent cations [39], In most cases the electrochemical techniques used were potentiometry and amperometry either under constant potential or in cyclic voltammetric regime. More recently, sensors for silver [29] and arsenite [40] on the basis of transition metal hexacyanoferrates were proposed. An apparent list of sensors for non-electroactive ions is presented in Table 13.1. 

Contrary to In3+, the heaviest d acceptor of group 3B, Tl +, is a very soft acceptor, as is evident from the stabilities of its chloride and bromide complexes (Table 2). The lower iodide complexes are not stable relative to the redox reaction producing thallium(I) and free iodine. The inherent affinity of T13+ for 1 is so strong, however, that even at rather modest concentrations of free iodide, thallium (III) is completely protected from reduction by formation of the complex Tlli (80). The value... 

Thallium(II) has also been invoked as an intermediate in the photochemical reduction of aqueous Tl111 solutions,353,354 but there is dispute as to its involvement in redox reactions.355,356... 

Other Thallium Compounds.—Tl2 ions may be generated by flash photolysis of Tl111 solutions.626 Some of their redox reactions have been studied, and they give values of the standard reduction potentials for the reactions ... 

Several redox reactions involving Tl(III) in aqueous solution have been extensively studied (see Section I and IV,B). Some of these have been proposed to proceed via Tl(II) as an intermediate, e.g., the reaction with Fe(II) (70,71). Therefore, to understand the mechanisms of such redox reactions, it has been of interest to ascertain the chemistry of thallium II). Thallium(II) can be produced in aqueous solution by reduction of TKIII) with hydrogen atoms or by radiolysis of aqueous solutions... 

The kinetic characteristics of ligand exchange of thallium complexes in solution have been little explored, in contrast to the numerous equilibrium studies (4-6,41,57,61,66,67,90,92,93,96,97,112,214-246) and a large number of studies of redox reactions involving thallium (81, 100, 103, no, 246-288). 

As mentioned in the introduction, the thallium(III)/thallium(I) redox couple has been widely used as a model system for electron transfer processes. There are numerous studies of both the electron transfer between two metal ions and the redox reaction between a metal ion and another ion or molecule, usually an anion. This interest has been well documented and reviewed in the past, but also in modern books dealing with electron transfer reactions 315,316). Therefore, and also because the large amount of experimental and theoretical data would call for a separate review article, this subject will not be discussed here except for a general comment and a discussion of a few very recent papers. 

Relatively little work has been done on the redox reaction between thallium and halide/pseudohalide ions (75, 93, 97,110, 328-330). Let us consider the qualitative order of stability of thallium(III) in the form of TlXp " complexes, where X = Cl, Br, I, SCN, CN, Thallium(III) forms strong complexes with all these ligands on the other hand, it can oxidize the X ions to X2. It is well known that the thallium(III) chloride complexes are perfectly stable for an indefinite period of time. The corresponding bromide complexes are usually stable, but at low Br/Tl ratios Tl(III) can be reduced by Br the reduction is easily prevented by adding excess of bromine. The iodo and thiocyanato complexes are approximately equally unstable toward redox reaction Tl(III) is rapidly reduced by the anion.Finally, the cyano complexes... 

Another type of thallium compounds which have potential applications in converting light into electrochemical energy are the hi- or oligometallics of the type shown in Fig. 19. Similar compounds, such as (CN)5-Fe -(/u-CN)-Pt (NH3)4-(/L-NC)-Fe -(CN)5, have recently been shown to undergo redox reaction when irradiated by visible light (356). Research in this field has been active during the last few years, and... 

The problems associated with the M state of the elements of group III have been noted previously (cf. Sections 25.1.13 and 25.2.3). The TP ion has been identified in pulse radiolysis decomposition of aqueous Tl2S04, and in y-irradiated frozen aqueous solutions. Thallium(II) has also been invoked as an intermediate in the photochemical reduction of aqueous Tl" solutionsbut there is dispute as to its involvement in redox reactions. It is safe to conclude that there is no equilibrium aqueous phase chemistry of TP" ". 

Redox reactions in acetonitrile have been used to prepare a number of coordination compounds of thallium(III), such as ... 

The compound TII3 is isomorphous with the alkali metal triiodides and is really thallium(I) triiodide, 13.17. However, when treated with excess I , an interesting redox reaction occurs with the formation of [Tll4] (see Section 13.9). The decrease in stability of the higher oxidation state on going from the binary fluoride to iodide is a general feature of all metals that exhibit more than one oxidation state. Eor ionic compounds, this is easily explained in terms of... 

Fluorometric methods are based on the formation of fluorescent species in redox reactions where oxidation of A A is produced. The fluorescent species can be o-phenylenediamine [95,96], quinoxaline [96-98], Leucothionine blue [99], and thallium(I) [100], where the violet fluorescence is attributed to the presence of the TlCli ion. 

Several other useful reviews of reactions involving metal ions have also been published. Redox reactions of chromium(m)-amine species have been described and a survey has been made of the solution chemistry together with reaction paths involved in the redox reactions of various plutonium species. Oxidation reactions of thallium(m) have also been described. Developments in the redox chemistry of peroxides have been reviewed, the nature of the reactions which involve iron(iii) in various complexed forms providing a fascinating example of the manner in which geometry and co-ordination to the metal centre greatly affect the reactivity of the system. Redox properties of cobalt chelates, with delocalized... 

Thallium.—The decomposition of pyridinomethylthallium(iii) in aqueous solution in the presence of chloride is a redox reaction since the inorganic products are thallium(i) species, but the important step is 5 n2 attack of chloride at the methylene-carbon, with synchronous displacement of the thallium. The determined activation parameters, Aif = 28 kcal mol and - + 14 cal deg mol- for reaction of TlCl(CH2pyH) + with elimination of TlCl, are consistent with an 5n2 mechanism. The decomposition of RTIX2 into TlX + RX, in pyridine solution, also proceeds by nucleophilic attack, by pyridine or halide, at carbon. ... 

The Heck reaction has now been reviewed448,449. Evidence for the formation of zerova-lent palladium from (AcO Pd and Ph3P via a redox process has been provided450. This explains the origin of Pd(0) required for certain palladium-catalysed reactions in cases where Pd(II) is added to the reaction as the primary form of the Pd-catalyst. Thallium has been found to accelerate the Heck-type cyclization-carbonylation451. 

In contrast to the other elements of group 13, thallium is considered a soft acid in Pearson s soft/hard acid classification (see Hard Soft Acids and Bases).This makes the element and its derivatives unique, and leads to its potentially most outstanding feature the properties of thallium are a subtle blend of some of the most desirable properties of numerous other metals (e.g. heavy alkali metals, silver, mercury, and lead). Thallium compounds are stable in both oxidation states (-1-1 and -1-3). The trivalent cation is quite a strong oxidation reagent, since it is reduced to T1+ easily (standard redox potential E°(TP+ — Tl" ") = -1.25 V). The ease of this reduction is utilized in certain organic reactions. 

Calculating The concentration of thallium(I) ions in solution may be determined by oxidizing to thallium(III) ions with an aqueous solution of potassium permanganate (KMn04) under acidic conditions. Suppose that a 100.00 mL sample of a solution of unknown T1+ concentration is titrated to the endpoint with 28.23 mL of a 0.0560M solution of potassium permanganate. What is the concentration of T1+ ions in the sample You must first balance the redox equation for the reaction to determine its stoichiometry. 

A homogeneous catalyst exists in the same phase as the reactants. Ceric ion, Ce4+, at one time was an important laboratory oxidizing agent that was used in many redox titrations (Section 11-8). For example, Ce + oxidizes thallium(I) ions in solution this reaction is catalyzed by the addition of a very small amount of a soluble salt containing manganese(II) ions, Mn +. The Mn + acts as a homogeneous catalyst. 

Propylene was similarly oxidized electrochemically to acetone and glycol in the presence of mercurous and thallium salts 334,335). It is noteworthy that increasing anodic potentials (> +1.8 V) favored cleavage of the propylene chain to yield formic and acetic acid. It is conceivable, then, that unexplored low anodic potentials or electrogenerative oxidation at 0.6-1.0 V could minimize or eliminate these undesirable side reactions. The significance of the nature of the redox couple and its concentration in solution for controlling reaction selectivity has already been emphasized earlier. 

The increasing interest in thallium chemistry is due to several distinct applications. Besides the above-mentioned redox properties used in organic preparative chemistry and for model studies of electron transfer reactions, it can be perceived that thallium has found applications as a model for the general behavior of metal ions. For example, Tr is of particular interest because of its importance as a probe for the role of alkali metal ions. The ionic radius of TL, rxl. = 1.47 A, is only slightly larger than ky. = 1.33 A, and approximately the same as rRb+ = 1.47 A (34). Because of the ease with which Tl can be monitored by spectroscopic (35, 36), fluorescence (57), polarographic... 

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