Thallium stability constants

The pseudohalides and related compounds have been investigated by a number of workers. Thallium(I) cyanate and thiocyanate are both known, and the latter has been shown to be ionic in the solid state 288 differing values have been reported for the stability constants in the T1/NCS system in aqueous or mixed water/non-aqueous systems, but the overall evidence is that the complexes are extremely weak.289 The ionic selenocyanate has been reported, but the chemistry has not been investigated.1... 

Stability constant studies show that in aqueous solution the anions T1FJ, T1F2- and T1F4 are formed 289 some of these may be aquated. Many double salts have been reported,1 but it seems that thallium is present in these as the Tl+ cation rather than as TLF0 I+,) anionic complexes. 

Literature values for stability constants (153) have usually been determined for solutions much less concentrated than those needed for diffraction measurements, and the values for these solutions have to be checked by other methods. For the thallium(III) bromide complexes the stability constants for the concentrated solutions used [1-2.6 M in Tl(III)], were derived from Tl-205 NMR shift measurements. The fraction of Tl(III), bonded in each of the complexes calculated from these constants as a function of the chloride concentration (Fig. 17), shows... 

Sutton (666) and Kul ba (458-460) have prepared a number of bipyridyl and phenanthroline complexes of various thallium(III) salts those with nitrate and perchlorate are generally bis-chelate compounds, whereas the halides give compounds of stoichiometry TIX3L. Stability constants have been reported for some of the complexes (457). Other mixed ligand species containing ethylenediamine (461) and oxalate (456) and salts containing both bipyridyl and phenanthroline coordinated to the same thallium(III) ion (456) are also claimed. 

A detailed, multimethod study of hydrated Tl(III) cyanide species in aqueous solution reveals that Tl(III) forms very strong complexes with cyanide ions (even stronger than halide-Tl(III) interactions)." " Formation of a series of Tl(III) complexes T1(CN) n= -4t) has been established, and the solution structures and stability constants were reported. The mono- and dicyano complexes [Tl(CN)(OH2)5] and [Tl(CN)2(OH2)4] show six-coordinate thallium centers, whereas Tl(CN)3(OH2) and [T1(CN)4] have four-coordinate T1(III) ions. 

Even if many of the stability constants of thallium(I) complexes in aqueous solution have been determined before the time period covered by this review (58), a selection of constants is presented in Table I (59-68), since they are of interest for investigating several other properties of thallium(I) complexes (see, for example. Sections III,A, IV,A,1, and V). Most of the known stability constants for the complexes of... 

Overall Stability Constants for Thallium(I) Complexes in Aqueous Solution at 25°C ... 

The most substantial investigation of thallium(II) species in aqueous solution is that of Dodson and Schwarz, who studied equilibria and kinetics of Tl(II)-Cr complexes (77). On the basis of some (reasonable) assumptions, they have calculated the stepwise stability constants K for the three T1C1 " complexes, n = 1,2,3, in 1M HCIO4 and estimated = 1 from the ratios between the stability constants K, for Tfi Cl " and TT C1 ". The absorption spectra of the individual Tl(II) chloride complexes have been derived from their stability constants in combination with the experimental spectra recorded for solutions with varying composition. Absorption maxima were found at 263 and 342 nm for TlCl, at 280 and 342 nm for TICI2, and at 304 and 362 nm for TlCla". An interesting observation was the 10-fold increase of the extinction coefficient (at —340 nm) of the Tl(II) solution in 1 M perchloric acid upon addition of a small amount of chloride ion ([Cl ] = 10 M) (77). 

The few known stability constants of Tl(II) complexes are collected in Table II (85, 86). When the values of the corresponding equilibrium constants are compared for Tfi, Tl , and Tl (cf. Tables I-III) it can be noted that the coordination of thallium(II) is intermediate between (T1(I) and Tl(III). The stability of the chloro complexes follows the order TfiCl " < TFC1 " acid strength of the hydrated thallium ions. Coulombic interactions of the differently charged metal ions are probably the main reason for this effect, but other properties (polarizability, lone-pair effect) certainly play a role. It is probably safe to assume also that the softness... 

M thallium solutions, containing 1 M NaC104 and 3 M HCIO4 as ionic medium, the obtained stepwise stability constant, = 0.3(1) is not higher than the detection limit given by Ahrland et al. in their potentiometric study (91, 92). It is also in acceptable agreement with K5 = 0.8(2) M obtained by spectrophotometric measurements... 

These spectral features and other evidence presented in this paper (97) leave no doubt that the Tl(III) cyano complexes exist, that they have the stated composition, and that they are extremely strong and kinetically inert. In fact, the cyano complexes are stronger than any other known monodentate complexes of thallium(III). The only possible known competitor as a ligand, the iodide ion, forms the complex TII4 with the overall stability constant log = 35.7 (99), i.e., several orders of magnitude lower than that of T1(CN)4" (see Table III). The distribution of thallium among the various Tl(CN), "-species is shown in Fig. 5. Stepwise stability constants of MX complexes often decrease with increasing n because of statistical, steric, and coulombic factors... 

There is some evidence that back-donation plays an important role in cyano complexes (149). For the isoelectronic Au(I), Hg(II), and Tl(III) ions, the back-donation should be most efficient (and hence the complexes should be strongest) for gold and least efficient for thallium because of the increasing charge on the metal ion. Thus, if back-donation is a major effect, the stability constants for the cyano complexes should decrease in the order Au > Hg > Tl. Unfortunately, only one stability constant, namely /32, is (approximately) known for gold(I), but there is no doubt that the 2-values follow the predicted trend log 82 = 39 (for Au) > 32.7 (for Hg) > 26.5 (for Tl) (97, 150,151). 

Table 6.8 gives stability constants for the complexes [FeX] and [HgX] for different halide ions while the stabilities of the Fe complexes decrease in the order F > CP > Br, those of the Hg complexes increase in the order F < CP < Br < P. More generally, in examinations of stability constants by Ahrland, Chatt and Davies, and by Schwarzenbach, the same sequence as for Fe + was observed for the lighter s- and /i-block cations, other early J-block metal cations, and lanthanoid and actinoid metal cations. These cations were collectively termed class (a) cations. The same sequence as for Hg complexes was observed for halide complexes of the later J-block metal ions, tellurium, polonium and thallium. These ions were collectively called class (b) cations. Similar patterns were found for other donor atoms ligands with O- and iV-donors form more stable complexes with class (a) cations, while those with S- and F-donors form more stable complexes with class (b) cations. 

Table VIIL The Stability Constants and Absorption Spectra of Some Thallium (I) Complexes at Ionic Strength 0.075"...

Thallium(i) chloro-, thiocyanato-, and chloro(thiocyanato)-complexes have been studied potentiometrically (using a T1 amalgam electrode) at 10, 25, 40, and 60 °C. Thermodynamic stability constants were calculated at these temperatures. Enthalpies and entropies of formation of TlCl, T1(NCS), and TlCl(NCS)- were evaluated from the temperature dependence of the stability constants. ... 

Ahy is independent of and that only mononuclear complexes are formed. This piece of information could not be obtained in the potentiometric determination of the stability constants (2) where mononuclearity had to be assumed in order to integrate the BODLANDER equation. The calorimettic titration has given proof that this procedure was justified. In the Tl -Br" system a point of equivalence was obtained at ca 4c v4 as practically no heat was evolved after this point. This indicates strongly that no more than four bromide ions can be bound by a thallium(III)-ion. This fact is also in agreement with the result obtained by the potentiometric method (2). 

Thiophen Derivatives of Analytical Interest.—2-Thenoyltrifluoroacetone has maintained its position as a chelating agent in analytical chemistry. Papers describing its use in the extraction or determination of thorium, copper, europium, thallium, niobium, and molybdenum have appeared. The effect of copper(n) on the formation of monothenoyltri-fluoroacetonatoiron(iii) has been studied. The stability constants of some bivalent metal chelates of di-(2-thenoyl)methane have been determined. 3-Thianaphthenoyltrifluoroacetone has been proposed as a reagent for the spectrophotometric determination of iron(iii) and cerium(iv). The stabilities of metal chelates formed from derivatives of thiophen-2-aldehyde and of rare-earth carboxylates of thiophen-2-carboxylate have been studied. 

CR represent different conformations of the crown ether. T1+ is closer to Ag+ than to K+ or Rb+ in its behaviour. In studies of the aquation of [RhCl,] " by Hg +, Tl +, Cd +, and In + and of the equation of [RhBre] " by Tl + there is a general correlation between the rate and the stability constant of the monohalogeno-complex of the catalytic cation. The decomposition of [MeTl(OAc)2] to TlOAc and MeCOaOMe has been studied in a variety of solvents. Usually simple first-order kinetics are observed, but in deuteuriochloroform and in THE there is an initial induction period in which the solution becomes saturated with TlOAc. The authors propose a rate-determining 5n2 reaction of [AcO] with the methyl group bonded to thallium. 

The equation for the average ligand number, as shown by Eq. (3.8), has been derived where only monomeric hydrolysis species form. As can be seen, the equation is independent of the metal ion concentration, and, as such, plots of average ligand number data against pH should be independent of the metal ion concentration. This behaviour is illustrated in Figure 3.3. The data shown in the figure are for potentiometric measurements to determine the stability constants of the hydrolysis species of thallium(III). This metal ion only forms monomeric species. 

For T1(0H)2, stability constant data have only been accepted from experiments conducted at 3.0 mol l , although these studies used both lithium and sodium perchlorate. The stability constant at zero ionic strength has been calculated using the ion interaction parameters found for iron(III) which, having the only appropriate data that are available, are likely to be closest to those that would be applicable to thallium(III). 

A number of studies have obtained stability constants for the hydrolytic reactions of thallium(III), but the data from one these, where a range of ionic strengths were studied, are not accepted by this review. All studies utilised perchlorate media, but data have only been accepted at essentially two ionic strengths, either 1.5 or 3.0 moll , with the latter having reported values where both lithium and sodium perchlorate have been used, the obtained constants being in reasonably good agreement. The reported stability constants, which are only for monomeric species, are listed in Table 13.29. 

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