Thallium stability

For the 2223 compound of the Tl-Ba-Ca-Cu-0 compound the loss of thallium is reduced by surrounding a pre-fired, and thallium-deficient, sample with powder of the compound and refiring. These substances also show a temperature-oxygen partial pressure stability relationship, similar to the classical 123 compound, but the details have not yet been elucidated. 

A number of alkylated and ring-annelated derivatives of cyclopentadienyl-thallium(I) have been reported. All were prepared by the same procedure used for the parent compound, and relevant experimental data are listed in Table I. None of these compounds is as stable as cyclopentadienylthallium-(I) the methyl-substituted derivative, for example, undergoes essentially spontaneous oxidation on exposure to the atmosphere (25), and, qualitatively, the order of stability has been assessed (105) as... 

The effect of structure of the alkyl group on the stability of monoalkyl-thallium(III) compounds can best be understood by reference to the different mechanisms by which these compounds undergo decomposition. A number of authors have attributed the instability of monoalkylthallium(III) compounds to facile C—T1 bond heterolysis and formation of carbonium ions [Eq. (25)] (52, 66, 79). This explanation is, however, somewhat suspect in cases where primary carbonium ions would be involved and either the two-step sequence shown in Eqs. (26), (27), or the fully synchronous 8 2 displacement shown in Eq. (28), is more compatible with the known facts. Examination of the oxythallation reactions that have been described reveals that Eq. (27) [or, for concerted reactions, Eq. (28)] can be elaborated, and that five major types of decomposition can be recognized for RTlXj compounds. These are outlined in Scheme 8, where Y, the nucleophile... 

Oxidation of the steroidal olefin (XXVII) with thallium(III) acetate gives mainly the allylic acetates (XXXI)-(XXXIII) (Scheme 15), again indicating that trans oxythallation is the preferred reaction course (19). Addition of the electrophile takes place from the less-hindered a-side of the molecule to give the thallinium ion (XXVIII), which by loss of a proton from C-4 would give the alkylthallium diacetate (XXIX). Decomposition of this intermediate by a Type 5 process is probably favorable, as it leads to the resonance-stabilized allylic carbonium ion (XXX), from which the observed products can be derived. Evidence in support of the decomposition process shown in Scheme 15 has been obtained from a study of the exchange reaction between frawr-crotylmercuric acetate and thallium(III) acetate in acetic acid (Scheme 16) (142). 

Organoelemental thallium(I) compounds were prepared by similar routes starting with thallium(I) cydopentadienide, for instance [Eq. (4)]. In contrast to the clusters described so far, the metal-metal interactions in the tetrahedral dusters of compound 19 [42] or of the related pyrazolato derivative 20 [43] (Figure 2.3-4) are quite weak their bonding situation and stability are discussed in Section 2.3.3.1.2. 

Thallium may be described as a relativistic alkali metal the downshift in energy of the 65 orbital, due to a combination of relativity and shell structure effect, favours the oxidation state I over III (see 4.2.22). The stability of the oxidation state +1... 

The thallium trinitrate-mediated ring contraction of frani-decal-2-ones has opened up a new route to the hydrindane system, and fluorinative ring contraction of cyclic alkenes to afford difluorocycloalkanes has been induced by iodotoluene difluoride and EtsN-HF. A possible mechanism is shown in Scheme 78. The double bond of the cyclohexene ring is attacked by iodotoluene difluoride activated by HF from the axial direction, followed by the addition of a fluoride ion from the trans direction. Reductive elimination of iodotoluene from the resulting adduct, ring contraction and the addition of the fluoride ion to the carbocation stabilized by fluorine then take place to give the ring-contracted difluorinated product. 

The lack of homopolyatomic anions for elements to the left of group IV In Table I is noteworthy. Zlntl reported no success with reactions of alkali metal alloys of the copper and zinc family elements and of thallium with liquid ammonia, and the generally stabilizing effect of crypt has not been evident In our own Investigations of alloys of mercury and thallium. On the other hand. It is possible to Isolate a white crypt-potassium gold compound from ammonia solutions at low temperatures which decomposes to elemental gold (+ ) above about -10°C (30). 

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 and lead have higher values of their first ionization energies than expected from the trends down their respective groups, because their p-orbitals are more compact. The relativistic effect upon the 6p-orbitals of the elements from Tl to Rn is to reinforce a stabilization of one orbital with respect to the other two. Instead of the expected trend, the first ionization energies ofTl, Pb and Bi ( + 589, +715 and +703 kJ mol- ) do not show a general increase like those of In, Sn and Sb (+558, +709 and + 834 kJ mol ) the value for Bi is lower than that of Pb. 

A cationic mechanism is responsible for the ring contraction of cycloalkenes with thallium(III) salts in the presence of diluted perchloric acid resulting in the formation of formylcycloalkanes. This method was unsuccessful for cyclopentene, whereas 1-methylcyclopentene gave acetyicy-clobutane (32) in 16-24% yield98 depending on the better stabilization of the intermediate cationic species. 

Long X-Tl (X = halogen) contacts between the thallium center and halogen atoms of the almost linear [AuR2] unit seem also to contribute to the stability of many of these systems. It is worth noting that theoretical studies revealed for the Au-Tl interaction in these systems a surprising calculated strength of about 276 kj mol-1, from which 80% is due to an ionic contribution and 20% to dispersion (van der Waals) [55]. 

For A, the nitrogen coordination is invariably planar because of a low to zero inversion barrier which facilitates possible overlap of the M and N p-orbitals. The n-overlap in B is facilitated by a linear geometry. However, a further consideration in the case of B is the increasing stability of the lower oxidation state of the metal when descending the group. This effect is most prominent in thallium and it implies an increasing stability of the monovalent unit R-M which may lead to destabilization or distortion of structure B. 

One of the classical properties of the main group elements is that the stability of the lower oxidation states increases with atomic number, and the chemistry of thallium is a good example of this effect. In aqueous solution, the Tl+ ion is stable with respect to oxidation by the solvent and there is accordingly an extensive chemistry of this oxidation state. The similarities between Tl+ and the corresponding alkali metal cations have resulted in much interest in the use of this ion as a probe in biochemical systems, and the ease with which 205T1 NMR spectra can be recorded has also had an impact on such studies.277,278... 

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